Deconstructing the Tower Defense Genre.
Giving Options to the Players.
A Supportive Narrative for Graduation Purposes at the
Utrecht School of the Arts
in Association With the Open University, London
Author: Newen Fara
Pathway: Games and Interaction
Course: Game Design & Development
Supervisor: Janine Huizenga
Year: 2013 - 2014
1
Abstract
In casual gaming, tower defense games have reached a ceiling on innovation; the
majority of new tower defense games are practically all the same with little variation,
except for graphics and story. The mechanics and interaction design is pretty much the
same.
This paper explores the mechanics of tower defense games and how they work by
analyzing tower defense games that present differences in mechanics. This analysis will
help understand how the mechanics of TD games can be modified and tweaked. How
balancing a current tower defense game works and how this system is affected when
changing the mechanics. What happens to this balance system when the player has
freedom to design towers? How the economy of a TD game works and how it may be
affected if players have freedom of choice.
Keywords
Tower defense, design, game balance, upgrade, freedom, towers, creeps, strategic dominance,
intransitivity, prototyping, Rock Papers Scissors, casual games, mobile games.
2
Table of Contents
1. Introduction
1.1. About this Document
1.2. About the Author
1.3. Motivation
1.4. Goal
1.5. Method
1.6. Terminology
2. What is a Tower Defense Game?
2.1. Definition
2.1.1. Definition 1
2.1.2. Definition 2
2.1.3. Definition 3
2.1.4. Definition 4
2.1.5. Short Definition
2.1.6. Expanded Definition
2.2. Base Mechanics
2.3. Extra Mechanics
3. What kind of game is a tower defense game?
3.1. Strategy vs. Real Time Strategy
3.2. Strategy vs. Puzzle
3.3. Who plays tower defense games?
4. Balance Elements of a TD Game
4.1. Basic elements
4.1.1. Creeps
4.1.2. Towers
4.1.3. Level Design
4.2. Extra elements
4.2.1. Creeps
4.2.2. Towers
4.2.3. Others
4.3. TD Strategy
5. Tower Upgrade
5.1. State of the art
5.2. Applying towers balance elements as upgrades
5.3. Price of upgrading
5.4. Keeping the stress throughout the game.
5
5
5
5
5
6
6
7
7
7
7
7
7
8
8
8
9
10
10
10
10
11
11
11
11
11
11
11
11
11
12
13
13
18
18
26
3
6. Strategy
6.1. What is a strategy
6.1.1. Definition of Strategy:
6.1.2. Dominant strategy.
6.1.3. Tower defense dominant strategy.
6.2. Intransitivity. Rock, Paper, Scissors.
6.3. Examples of using the balance elements as strategy.
6.3.1. Range.
6.3.2. Damage.
6.3.3. Fire rate.
6.3.4. Damage conditions
6.4. Adding types as a new balance element
7. Research
7.1. Prototypes
7.1.1. Prototype 1
7.1.2. Prototype 2
7.1.3. Prototype 3
7.1.4. Prototype 4
7.2. Questions
7.2.1. General questions
7.2.2. Questions for prototype 1
7.2.3. Questions for prototype 2
7.2.4. Questions for prototype 3
7.2.5. Questions for prototype 4
7.2.6. Final general questions
7.3. Results gathered
7.3.1. General questions
7.3.2. Questions for prototype 1
7.3.3. Questions for prototype 2
7.3.4. Questions for prototype 3
7.3.5. Questions for prototype 4
7.3.6. General questions 2
7.4 Analyzing the data
8. Conclusion
9. Bibliography
27
27
27
27
27
27
28
28
29
30
31
32
33
33
33
34
35
36
36
37
38
39
40
41
42
43
45
46
48
49
51
54
4
1. Introduction
1.1. About this Document
This document is the final assignment of my Game Design and Development studies. It is a one
year Masters degree. My focus is on video game design for mobiles. This is facilitated by my
experience as a flash and web developer to develop the game this paper will support.
1.2. About the Author
I have always been inspired by existing things. I marvel at the ingenuity of people to invent, and
at the same time, cannot help but think on how to improve upon them. It is in spirit of making
things better, not because they are broken, but because they can be better. Be it a web
development workflow, or a game design. I focus on analyzing things, situations and processes
to understand them and find where they can be optimized or modified to make them better.
1.3. Motivation
This project was born from a search for freedom. I’ve loved tower defense games from the first
time I played Desktop Tower Defense (Kongregate n.d.). After playing many different tower
defense games I am left with a hunger for a more customizable game, a game where I can
design my own towers and test different strategies. A game where I decide how to attack the
creeps, not just choose a tower from a list. A game that does not limit my ability to come up with
different strategies.
1.4. Goal
So here I am, searching for a way to give freedom to the player when it comes to building
towers. Can you imagine building a cannon tower and then adding to it a pinch of poison
damage for that “damage over time” (DoT) kill? Or a slow tower that also spews acid on its
victims dealing extra damage plus DoT, I am not saying it is going to be a cheap upgrade, mind
you. But imagine towers that can be built as if you are a war engineer, taking different elements,
modules and parts and mixing them to come up with that tower that you had in your head!
That is my goal. But before we get there, we have to make sure that a game like this can work,
that the mechanics stay true and it can be balanced. How to make this kind of game so it works
and is fun for players. That is the goal of this paper.
By means of this paper I will explore how this freedom can be achieved and what effect it will
have in the mechanics and balance.
5
1.5. Method
To achieve this goal I will focus on answering this question:
“How does giving players the freedom to design their own towers, affect the
mechanics of tower defense games?”
And to answer this question we need to delve deeper with some sub questions.
- What is a tower defense game?
- What are the balance elements in tower defense games?
- How to make tower upgrading non-linear to allow adapting to different scenarios?
- How to balance the tower elements to avoid a dominant strategy?
- How is the emotional effect changed from current tower defense games.
1.6. Terminology
Tower: Static unit or building that automatically attacks creeps.
Creep: Enemy unit.
Entrance: Entry point of the map where creeps come in.
Exit: The point where the creeps exit the map.
Lives: Unit of measure for winning the game.
TD: Tower defense.
DoT: damage over time.
AoE: Area of effect.
6
2. What is a tower defense game?
2.1. Definition
Here I will explore some definitions of what a tower defense game is and compare them to find
a more complete definition.
2.1.1. Definition 1.
“Tower defense is a subgenre of real-time strategy video games. The goal of tower defense
games is to try to stop enemies from crossing a map by building traps to slow them down and
towers which shoot at them as they pass. Enemies and towers usually have varied abilities,
costs, and upgrade prices. When an enemy is defeated, the player earns money or points,
which are used to buy or upgrade towers, or upgrade the number of money or points that are
earned, or even upgrade the rate at which they upgrade.” (Wikipedia n.d.)
2.1.2. Definition 2.
“Tower Defense (TD) games are a genre of strategy games focusing on resource allocation and
unit (tower) placement. In its simplest form, a TD game consists of a human player buying and
organizing defensive towers that fire upon a set deployment of different types of offensive
enemies (creeps).
For each creep destroyed by the towers, the player earns resources corresponding to the
difficulty of the creep. If enough of the creeps are destroyed, the player wins the round. If
however enough creeps reach the end of the player’s map, the creeps win. The tower
deployment usually involves buying, placing and upgrading towers that fire automatically on the
forces.” (Avery et al. 2011)
2.1.3. Definition 3
“Tower defense (TD) games ask players to protect a base by organizing the base’s defenses.
The player is given a map (typically a simple maze) showing the path the enemies (generically
referred to as creeps) will follow through the maze and a budget for purchasing defense towers
to be placed on the walls of the maze.
The player cannot directly interact with the creeps. The only choices the player has are which
types of towers to use and where to place them. Towers have a fixed rate of fire, do a fixed
amount of damage per shot and never miss. Creeps move at a fixed speed and never veer from
the path. The score in the game is based on the number of creeps stopped.
Although the creeps are nominally the enemy, they are deterministic and do not react to the
player. As such, TD is technically a puzzle, not an adversarial game.” (Wetzel et al. 2012)
2.1.4. Definition 4.
“Tower defense games consist of a number of relatively simple parts. Enemies follow a
predesigned path toward the player’s fortress. Each enemy has a particular speed, a number of
hit points, and perhaps a few attributes to make it more interesting. The player places towers to
defend his position. Each tower fires projectiles at enemies within a certain range and at a
7
certain rate. Some towers deal damage while others produce other effects, such as slowing
enemies down. Sometimes towers will boost the performance of neighboring towers.”
2.1.5. Short Definition.
Looking for the shortest description of what a TD game is, I come to this definition:
“In a tower defense game the player attempts to stop enemies from moving across a map by
placing static defensive units at strategic locations.”
2.1.6. Expanded Definition.
Looking for a description that includes all basic mechanics, I come to this definition:
“A tower defense game is a strategy game where the player positions fixed units (towers) that
will automatically attack enemy units (creeps) in order to kill them before they traverse the map.
The path creeps take may be fixed or dynamic, changing by placement of towers. The strength
of the creeps grows with every wave so the difficulty is ever increasing. Towers are built using
money, which is acquired by killing creeps. A match may have winning conditions, like surviving
X number of waves, or it may be endless and last until the player loses by not killing the creeps
before they cross the map. Towers can be upgraded to make them stronger so they can deal
with stronger creeps.”
2.2. Base Mechanics
Based on the previous definitions of what a TD game is, here is a list of base mechanics that
describe what a TD game is. This list is not fixed; a TD game may have all or some of these
mechanics. These would be the basic mechanics.
- There is a map with at least one entrance and one exit.
- Creeps will come in through the entrance and go out through the exit.
- The goal of the creeps is to get to the exit.
- The mission of the player is to stop creeps from reaching the exit.
- The player puts towers on the map to destroy the creeps.
- There should always be at least one path that allows creeps to reach the exit.
- When a creep reaches the exit the player loses one or more lives.
- The game is lost when the player has no lives left.
- Creeps usually come in turns or waves.
- The game is won when there are no more waves.
- Towers can be upgraded. *
- Building towers and upgrading them costs money. (Or whatever ‘currency’ is used)
* Upgrading towers is not a requisite of tower defense games, but as this paper deals directly
with this mechanic it is included in the basic list.
8
2.3. Extra Mechanics
These extra mechanics are also found in tower defense games around the web and mobile
devices. I include them here to recognize them as mechanics used in TD games and to make it
easier for others to categorize these types of games.
Depending on the game characteristics some mechanics may vary. Not all of these mechanics
are always found in a TD game. The game designer may choose to include or not any or all of
these mechanics into his game.
- Towers have predefined building spots.
- The path creeps walk is fixed and doesn’t change.
- The path is created by the placement of towers, creeps have to go around them.
- Creeps may be weak or strong against a specific type of damage.
- Towers have a damage type.
- The player has special attacks or powers.
- There is a hero/avatar on the map that works as a mobile defensive unit.
- Traps may be placed along the path.
9
3. What kind of game is a tower defense game?
3.1. Strategy vs. Real Time Strategy.
Strategy games are not as frantic as real time strategy (RTS) games. Tower defense games are
more paused; the success of the player is not defined by the amount of actions he can perform
by second or minute (as seen on Blizzard's RTS Starcraft2). As in all strategy games, the
success of a player in tower defense games depends strongly on the order of actions taken, but
the timing of these actions is not especially important. It may affect the game but it is usually not
the determining factor of winning or losing, like it is in an RTS.
More importantly, in RTS games the conditions of the game are constantly changing depending
on the opponents’ actions. In TD games, the opponent would be the creeps, but the creeps
usually come out always in the same order and do not react to the players actions, so after
playing the game the first time the player already knows in which order the creeps will come and
can plan accordingly, removing the real time element from the game.
3.2. Strategy vs. Puzzle.
“The score in the game is based on the number of creeps stopped. Although the creeps are
nominally the enemy, they are de-terministic and do not react to the player. As such, TD is
technically a puzzle, not an adversarial game.” (Wetzel et al. 2012)
Knowing that the player can plan ahead and study the situation on a map-by-map basis. The
game also resembles a puzzle, in as the player has to figure out the correct towers to build, and
where to build them in order to defeat the incoming waves of creeps.
3.3. Who plays tower defense games?
Tower defense games are easy to clone and do not have a big production cost.
The rules are simple and they are usually not very long; they can be played for short bursts of
time. You can play a TD game for as long as you want; there is no end.
TD is a casual game. (Portnow n.d.)
So who plays tower defense games? Casual gamers who like strategy and/or puzzle games.
10
4. Balance Elements of a TD game
Balance elements are the stats and/or rules of the game that a game designer can tweak,
change or manipulate. This is done to make a game more or less challenging. A common
example is the difficulty level in games.
4.1. Basic elements
This is a list of the mechanics as seen in the analysis of the games in chapter 4.3.
4.1.1. Creeps
- Life. A number representing health points.
- Speed. The speed at which the creep moves along the path.
- Number of creeps. How many creeps come in the wave.
- Interval. Time between the spawning of each creep.
4.1.2. Towers
- Damage. The amount of health a creep loses when hit by a tower.
- Firing speed. Amount of time it takes a tower to reload and be ready to fire again.
- Range. Maximum distance that a tower can target a creep.
- Damage conditions. Whether the tower attacks one or several creeps at the same time.
Single damage, splash damage or damage all around.
4.1.3. Level Design
- Map size. width by height in nodes. Measurable in grid based games.
- Path length. This affects the amount of time a creep may be vulnerable to damage.
- Available building spots and positions. Defines the maximum amount of towers that can
be built, thus defining the max amount of damage that can be done in the level.
- Fixed or dynamic paths. Determines if the path can change by tower placement or if
the path is always the same.
4.2. Extra elements
4.2.1. Creeps
- Weakness against a specific damage type. Like magic, poison or plasma.
- Strength against a specific damage type. Like magic, poison or plasma.
- Bosses. Creeps that are much more stronger than normal creeps.
4.2.2. Towers
- Specific damage type. Like magic, poison or plasma.
- Combo damage. When certain towers are built next to each other, an extra damage
occurs.
4.2.3. Others
- Heroes. A player-controlled mobile unit.
11
- Traps. Defensive, can be consumable items or buildable units.
- Level effects. May be environmental and automatic or triggered by special actions.
- Spells, powers or artifacts. Special player powers.
4.3. Common strategies
-
Building in corners.
Increasing the time creeps are under fire. Slowing creeps while fired upon.
Mazing. Making the path longer.
Mazing. Making towers attack different parts of the path. (switch backs)
Looping. Making creeps go from one side to the other and change the path to make
them retrace their steps. *only works in maps where the path is dynamic.
12
5. Tower Upgrade
5.1. State of the art
Case 1. Fieldrunners2. (IOS) (Apple 2014b)
In FR2 there are many different types of towers, the player selects a maximum of six towers to
play each level before starting. On the map there is a menu with the selected towers, from this
menu the player drags the tower he wants to build and places it where he wants it to be built.
The player can tap an existing tower on the map to get the tower options, then tap the upgrade
button to upgrade the tower. Each tower in FR2 has three levels which translates to building the
tower at level one and then upgrading it two times to get to the max level.
Fig. 1. Fieldrunners2 upgrade design.
13
On the tower options there is also an info button, which when selected, shows the damage info
for this tower at the current level. The info shown is usually the amount of damage per second.
FR2 presents maps where the path adapts to where towers are placed, and maps where the
path is set. Towers can never completely block the path.
Case 2. Carrot Fantasy. (Web,IOS) (Apple 2014a)
CF maps are grid based. To build a tower the player must tap on a grid square to get a menu
that shows the available towers for the current map. Selecting a tower from the menu will build a
level one tower. Each tower has three different levels and after building/placing the tower, the
player taps the tower again to get the tower options: all towers can be upgraded twice to get to
the max level.
The path is always predefined in all maps and is not affected by towers in any way.
Fig. 2. Carrot Fantasy upgrade design.
Case 3. Elf Defense. (IOS)
Elf Defense interaction and upgrade design is the same as in Carrot Fantasy. Tap on the grid to
get a menu, select the tower to be built, tap again to get options, upgrade. Towers have 4 levels
total. Building at level 1 plus 3 upgrades for max level.
The path is always predefined in all maps and is not affected by towers in any way.
Fig. 3. Elf Defense upgrade design.
Case 4. Kingdom Rush Frontiers. (Web, IOS, Steam) (Apple 2014c)
Kingdom rush is not grid based. Maps are free form, paths are always predefined and do not
change. To build a tower the player must tap on a building spot. These building spots are
usually in strategic points along the path. Tapping the build spot shows a menu with four towers
14
(Step 1), each tower has three basic levels. Building the tower at level one, then upgrading
(Step 2-3) twice to get to level three. After level three instead of one upgrade button the options
show two upgrade choices (Step 4). At this point, when choosing one of the two upgrade
options the tower changes to reflect what was chosen, and tapping the tower now gives a new
menu that shows two or three upgrade options. Each of them having two or three levels to
upgrade (Step 5).
Building Spot
Step 1
Step 2-3
Step 4
Step 5
Fig. 4. Kingdom Rush Frontiers upgrade design.
Kingdom Rush Frontiers has a total of 12 different towers. Four base towers, and eight pro
towers: two pro towers for each base tower. A pro tower can only be built from a level three
tower.
Case 5. Pirate Legends TD. (IOS) (Apple 2014d)
Pirate Legends TD plays exactly as Kingdom Rush Frontiers. Tap a predefined building spot to
get a four towers menu, tap the tower to build it and then tap the tower to get tower options and
upgrade it. After the third level choose between two pro towers and then 2 options of upgrades
for the pro tower with 3 levels each one.
Fig. 5. Pirate Legends TD upgrade design.
Case 6. Royal Defense. (IOS) (Apple 2013)
In Royal Defense we can see, again, the same basic user interaction for building and upgrading
towers as the one found in Kingdom Rush Frontiers and Pirate Legends TD, But there are no
pro towers. Tap a predefined building spot, get 4 options, then tap the built tower to get the
tower options and upgrade The path is also fixed and can not be changed. The difference lies in
that on the tower options we can choose to upgrade the tower to level three, skipping level two.
The price of the upgrade is the same as upgrading to level two and then upgrading from level
two to level three.
15
Fig. 6. Royal Defense upgrade design.
Case 7. TapDefense. (web, IOS) (Apple 2012)
TapDefense is grid based and has a predefined and fixed path. There is a general menu on the
bottom of the screen, from which one option is “Build”, pressing it opens a new screen with a
selection of towers.
When a tower is selected from the menu, the screen goes away and we are taken back to the
map with the new tower in the middle of it. Now the player can drag the tower to the desired
position. Towers cannot be placed on the path.
Fig. 7. Tap Defense upgrade design.
Case 8. GemCraft. (web, IOS) (Apple 2011)
GemCraft shows a somewhat different approach. Towers are made of two different parts. A
brick tower, which serves as a base for placing the second part of the tower; a gem. Gems are
what actually attacks creeps, so to build a tower the player must first build a tower, to then
create a gem, or select it from the library, and place it on the tower. There are different colors for
gems, each representing the type of damage it does.
There is no actual upgrading in this game. Gems are bought from a price list. The price of the
gem defines the level, stronger gems obviously being more expensive. So the player can buy as
16
high a level as his currency allows. The level of the gem is represented by the shape it has
(Square, triangle, rhomboid, pentagram, etc.).
Alternatively gems can be mixed and matched. Two gems of the same or different color can be
combined to create a new one. The new gem is stronger and has abilities from both gems that
were used to create it. Combining two level one gems will create one level two gem.
Fig. 8. GemCraft upgrade design.
Conclusion.
Most games implement towers with three of four levels. The first level being the placement of
the tower on the map, and then an upgrade button to increase the tower’s level, level by level
until the maximum level is reached.
In Kingdom Rush Frontiers we find something a little different. After the third level of each tower,
the tower can be upgraded to one of two pro towers and this tower has some upgrading options
that do not follow the one button upgrade mechanic. Another game that implements a different
upgrading mechanic is GemCraft, where the player can buy a high level gem directly, without
having to upgrade level by level after buying a level one gem.
So far all this upgrades are predefined options and fixed by the game designer. The freedom of
the player consists of choosing to buy a tower and where to place it, or upgrading an existing
tower. When a tower is upgraded the player has no control over what is upgraded or how that
affects the selected tower.
After further analyzing the experience around the upgrading of towers in Kingdom Rush
Frontiers, I can see how the designers mixed the simplicity of one button upgrades with the
complexity of upgrading options. It is a smart way to go about it. First the player upgrades the
“simple” way, with the one button upgrading, and when they are familiar and at ease with this,
they are introduced to the pro towers that have different upgrade options. These options may be
a balance element or extra mechanics placed by the game designer.
17
5.2. Applying towers balance elements as upgrades
If we treat balance elements as upgrade options we get: damage, range, firing rate and damage
type as independent options we can use to upgrade the tower. This allows the player to choose
between four options and upgrade what they want, depending on the strategy they want to
implement.
On top of the four basic elements we can add to this list specific damage types and creep
effects such as slowing the enemy or adding extra damage as a damage over time effect.
5.3. Price of upgrading
Here I will explore the mathematics behind some of the analyzed games to understand the
economy of upgrading. Comparing towers that have the same damage effect, we can see that
the maximum amount of damage a tower can inflict is usually determined by the initial price,
high cost towers being the strongest of them.
Using the Price of a tower and the damage it inflicts we can find the Price to Damage ratio.
Elf Defense.
Elf defense only shows price of upgrades. It does not give any information on amount of
damage or health of creeps.
Samson:
Icer:
Sharpshootr:
Stapler:
Cost, 10. Upgrades, 30 / 60 / 140 / 300. Ground and air.
Cost, 35. Upgrades, 60 / 100 / 150 / 300. Ground.
Cost, 100. Upgrades, 250 / 500 / 1200 / 2400. Air.
Cost, 120. Upgrades, 250 / 500 / 1200 / 2400. Ground.
* technical specifications taken from the iPhone game. Not all towers of the game are included.
We can see that almost always the price to upgrade a tower is more than double the initial cost,
then each upgrade is usually twice or more compared to the previous level.
The exception is the Icer tower.
Carrot Fantasy.
Like Elf Defense, Carrot Fantasy only shows price information for upgrading towers. No info on
creeps or towers damage. Towers with different attack effects are included but not all towers
from the game are listed, some towers of the game have the same building and upgrade cost.
Bottle:
Cost 100. Upgrades, 180 / 260.
Mud:
Cost 120. Upgrades, 220 / 260.
Fan:
Cost 160. Upgrades, 220 / 260.
Magic Ball:
Cost 160. Upgrades, 320 / 480.
Gas Bottle:
Cost 160. Upgrades, 260 / 320.
Frost Star:
Cost 180. Upgrades, 260 / 320.
*Information taken from the iPhone game.
18
In carrot fantasy the upgrades are cheaper compared to the initial cost and previous level
upgrades. There is no case where the upgrade is twice or more the initial cost. The price
increase in upgrade from level 2 to level 3 is usually less than the increase from level 1 to level
2. Only two towers have a constant increase in price, Bottle and Magic Ball, it is important to
note that these two towers are single attack damage, the other towers in the list all have area of
effect damage. This might be the reason in the difference in price increase from level to level.
Fieldrunners2.
Cost is always in gold coins, the currency used in Fieldrunners2.
Damage is always per second, ordered Level one / Level two / Level three.
Gatling:
Machine Gun:
Cannon:
Tesla:
Railgun:
Cost 5. Upgrade 4. Damage 35 / 70 / 105.
Cost 8. Upgrade 4. Damage 70 / 100 / 130. Has slow effect.
Cost 30. Upgrade 25. Damage 100 / 200 / 300.
Cost 70. Upgrade 50. Damage 700 / 1400 / 2100.
Cost 120. Upgrade 90. Damage 650 / 1333 / 2000. Has very big range.
* technical specifications taken from the iPhone game. Not all towers of the game are included.
Fieldrunners2 does give more information on towers, here we also have damage inflicted which
allows us to make more calculations.
We can see that the price for upgrading a tower is constant, as is the amount of damage
increased.
To compare the cost of having more damage in a tower we must compare how much damage
we get for each gold coin spent to buy or upgrade a tower across different towers. Dividing the
amount of damage inflicted by a tower when built and then when upgraded to level two and
level three by the amount of gold coins used to buy and upgrade said tower will give us the
damage / cost ratio at each different level.
Let us look at the amount of damage that is bought when placing a new tower and when
upgrading towers to a higher level.
Tower
Level one
Level two
Level three
Gatling
35 / 5 = 7
35 / 4 = 8.75
35 / 4 = 8.75
Machine Gun
70 / 8 = 8.75
30 / 4 = 7.5
30 / 4 = 7.5
Cannon
100 / 30 = 3.3
100 / 25 = 4
100 / 25 = 4
Tesla
700 / 70 = 10
700 / 50 = 14
700 / 50 = 14
Railgun
650 / 120 = 5.4
683 / 90 = 7.6
667 / 90 = 7.4
The data shows that upgrading a tower is more cost effective than building a new tower of the
same type, given that the upgrades are usually cheaper than the initial cost and the increase in
damage is usually the same. All towers have a constant increase in damage except the Railgun
tower.
19
Now let us look at how the ratio changes if we add the damage and prices along the upgrade.
Tower
Level one
Level two
Level three
Gatling
35 / 5 = 7
70 / (5 + 4) = 7.8
105 / (5 + 8) = 8
70 / 8 = 8.75
100 / (8 + 4) = 8.3
130 / (8 + 8) = 8.1
Cannon
100 / 30 = 3.3
200 / (30 + 25) = 3.6
300 / (30 + 50) = 3.75
Tesla
700 / 70 = 10
1400 / (70 + 50) = 11.6
2100 / (70 + 100) = 12.35
Railgun
650 / 120 = 5.4
1333 / (120 + 90) = 6.3
2000 / (120 + 180) = 6.6
Machine Gun
As expected the ratio goes up when upgrading a tower, making it more cost effective to upgrade
towers, than to buy new ones.
Taking a good look at this numbers we can see that the tesla tower has the highest damage of
all towers and the highest damage / cost ratio. All towers except the Machine Gun show an
increase in ratio when upgrading to a higher level. It is also evident that there is a big difference
in ratio between some towers, the Tesla tower showing 10 or more compared to the cannon,
which has maximum of 3.75. With all other towers in between.
Kingdom Rush Frontiers.
Kingdom Rush Frontiers also uses gold coins as the currency.
Damage is measured by amount of damage by hit. Range may differ from tower to tower. The
archer tower has a higher rate of fire than the other towers.
Archer tower level one: Cost, 60. Damage: 6 - 8. Average: 7.
Archer tower level two: Cost 100. Damage: 9 - 14. Average: 11.5
Archer tower level three: Cost 150. Damage: 13 - 19. Average: 16.
Mage tower level one: Cost, 100. Damage: 10 - 19. Average: 14.5.
Mage tower level two: Cost 160. Damage: 26 - 48. Average: 37.
Mage tower level three: Cost 240. Damage: 44 - 82. Average: 63.
Artillery tower level one: Cost 125. Damage: 7 - 14. Average: 10.5.
Artillery tower level two: Cost 220. Damage: 20 – 39. Average: 29.5.
Artillery tower level three: Cost 320. Damage 33 – 66. Average: 44.5.
To find a fixed number for the ratio I will use the average number of damage, instead of the
range.
20
Tower
Level 1
Level 2
Level 3
Average
Archer
7 / 60 = 0.116
11.5 / 100 = 0.115
16 / 150 = 0.106
0.112
Mage
14.5 / 100 = 0.145
37 / 160 = 0.231
63 / 240 = 0.262
0.173
Artillery
10.5 / 125 = 0.084
29.5 / 220 = 0.134
44.5 / 320 = 0.139
0.119
Knowing that most strategies are a combination of all towers we can take the average ratio of all
towers to find a general damage ratio, which would be 0.134. For each gold invested in towers
or upgrades, the tower power increases 0.134 damage points.
Creeps that make an appearance in level two, “Sandhawk Hamlet”.
Creep
Health
Strength
Speed
Bounty
Resistance
Ratio
Desert Thug
60
2-6
Medium
5
None
12
Dune Raider
200
6 - 10
Medium
16
Low Armor
12.5
Sand Hound
35
1-3
Fast
5
None
7
Immortal
360
12 - 28
Slow
24
Medium Armor
15
War Hound
120
12 - 18
Fast
10
Medium Magic
12
Sand Wraith
1000
30 - 60
Slow
100
None
10
Fallen
110
12 - 28
Slow
0
None
0
Dune Terror
100
4-8
Medium
10
None
10
Executioner
2000
30 - 60
Slow
50
None
40
*Creeps life and strength. Names, health strength, speed, bounty and resistance information taken from
the iPad version. Added the ratio column.
The ratio is a representation of the amount of damage needed to inflict to get one gold. Creeps
with a lower ratio are more lucrative than creeps with higher ratios. The difference means that
more gold is received for dealing the same amount of damage.
The following information was taken from a Kingdom Rush Frontiers wiki page at,
http://kingdomrushtd.wikia.com/wiki/Kingdom_Rush_Wiki
It was compared to the iPad in-game information and found correct and I added the total health
and wave ratio information to make it a more informational table.
Wave information for level two, “Sandhawk Hamlet”.
Difficulty level: Easy. Initial gold: 450.
21
Kingdom Rush has multiple lane map designs. Meaning that not all creeps come from the same
direction, which in turn makes the player have to manage where he builds towers for maximum
damage. I am not taking that into this analysis, therefore the amount of creeps for each wave
has been collapsed into one column.
Waves information:
Wave
Enemies
Income
Total Health
Wave Ratio
1
Desert Thug x20
100 G.
1200
12
2
Desert Thug x30
150 G.
1800
12
3
Desert Thug x23
Dune Raider x8
243 G.
2980
12.3
4
Sand Hound x38
190 G.
1330
7
5
Desert Thug x9
Dune Raider x4
Sand Hound x20
209 G.
2040
9.8
6
Desert Thug x26
Dune Raider x 10
290
3560
12.3
7
Sand Hound x 60
300
2100
7
8
Sand Hound x40
Desert Thug x10
Dune Raider x10
410
4000
9.75
Looking at the total health of all the creeps of a level we can have an idea of how difficult that
wave is. I am not taking into account other factors like speed of creeps here, though they do
affect strategy for defeating the creeps of each wave. We can see an increase in total health
from one wave to the next, with the exception of waves that have fast creeps, in this case Sand
Hounds. The ratio in this table shows how much damage must be dealt to get one gold. We can
see that the ratio is not constant through the waves. Looking at the data we see that waves that
are composed of only fast creeps have a ratio of 7, waves with both fast and normal creeps
have a ratio of around 9.8 and waves that have only normal speed or slow creeps have a ratio
of around 12.
If we take the total creep health per wave as one parameter and compare the potential tower
power, we get the following graph.
The potential tower power is the accumulated amount of gold multiplied by the general tower
power ratio number we found previously in this document (0.134).
22
Because gold should already be in possession of the player in order to be invested and used as
tower power, we start with the initial gold for wave one, then add the bounty from wave one to
find the power tower for wave two, and so one with each wave. Keep in mind that this ratio is an
average number and the actual tower power will vary depending on the strategy used by the
player and if all creeps of the wave are killed.
Curves:
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Wave 1
Wave 2
Wave 3
Wave 4
Wave health
Wave 5
Wave 6
Wave 7
Wave 8
Potential tower power
As what we are looking for is the behavior of the curves for total health per wave and potential
tower power, to study if and what relationship can be found between them, the general tower
power was multiplied by 10 so that the curves can be better compared.
One thing to keep in mind is that the towers damage data is per hit, not per wave. One tower
shoots several times during one wave. As long as there is a creep in range it will continue firing.
We can see that the curve for total wave health is not constant, it goes higher and lower at
different moments, but overall it has an upward slope.
The curve for tower power is much more constant and has an almost steady slope upwards.
If we took only the first and last wave as data, the slope would be very similar with the tower
power going a little bit higher over time compared with the total wave health.
To corroborate this data let us do this exercise again with a longer level.
Level 15th, “Emberspike Depths”:
Difficulty level: Hard. Initial gold: 1900.
Creeps found on level 15th:
23
Creep
Health
Strength
Speed
Bounty
Resistance
Ratio
Saurian Broodguard
240
8 - 22
Slow
20
None
12
Saurian Darter
200
12 - 22
Fast
20
None
10
Saurian Myrmidon
640
16 – 34
Slow
50
Medium Armor
12.8
Saurian Blazefang
480
18 – 22
Slow
40
High Magic
12
Saurian Razorwing
80
1–2
Medium
10
None
8
Saurian Nightscale
280
28 – 42
Medium
25
Medium Magic
11.2
Saurian Brute
3520
60 – 120
Slow
200
None
17.6
Saurian Savant
800
34 - 66
Slow
100
Medium Magic
8
Saurian Quetzal
400
None
Fast
100
None
4
Waves information:
Wave
Creeps
Income
Total Health
Wave Ratio
1
S. Broodguard x 16
320
3840
12
2
S. Broodguard x 10
400
4400
11
460
5280
11.48
500
6080
12.16
700
6840
9.77
760
9120
12
S. Darter x 10
3
S. Darter x 8
S. Myrmidon x 2
S. Broodguard x 10
4
S. Broodguard x 20
S. Myrmidon x 2
5
S. Broodguard x 12
S. Myrmidon x 4
S. Blazefang x 4
6
S. Broodguard x 26
S. Blazefang x 6
7
S. Razorwing x 28
280
2240
8
8
S. Broodguard x 14
880
10080
11.45
1000
10720
10.72
1000
11040
11.04
1500
23040
15.36
S. Nightscale x 24
9
S. Brute x 2
S. Broodguard x30
10
S. Broodguard x 30
S. Savant x 2
S. Nightscale x 8
11
S. Myrmidon x14
24
S. Brute x 4
12
S. Broodguard x 60
1950
24000
12.3
1850
19200
10.38
2400
30240
12.6
3080
43840
14.23
S. Myrmidon x 15
13
S. Darter x 40
S. Blazefang x 10
S. Savant x 4
S. Myrmidon x 5
14
S. Razorwing x 10
S. Quetzal x 4
S. Myrmidon x 14
S. Blazefang x 10
S. Brute x 4
15
S. Myrmidon x 8
S. Blazefang x 12
S. Brute x 8
S. Savant x 6
Curves:
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
Total health
Potential tower power
Data shows that on a harder map the curve shows a steeper angle on later levels.
We can only compare the first eight waves with “Sandhawk Hamlet”, because that level only has
eight waves. And the curve for those waves are comparable, the slope is rather similar. The
change is visible in waves that come after wave eight, where the slope turns upward.
25
We can see a difference in the fact that on the easy level, the curve for tower power had a
bigger change than the creep health per wave curve. What this means is that the tower power
has a potential to increase more than what the health per wave increases (percentage).
Eventually, the game would turn easy and the challenge would be lost out of excess of gold.
In the hard level we see the opposite, the curve for creep health increases more than the
potential tower power, the more the game progresses the harder it is.
The player has to make better use of the gold he acquires, invest it in strategic towers to
increase the tower power and deal with the increased creep health.
Another thing that is notable is that the balancing of the game is a matter of economy. How
much gold the user gains by killing creeps and how much tower power he can get with that
money. Is the gold won enough to acquire the tower power necessary to deal with the next
incoming wave? And repeat the process again?
5.4. Keeping the stress throughout the game.
We have seen how the difference between the slope of creep health per wave and potential
damage power shows us the difficulty level of a map. Keeping the player focused on the game
means keeping these two things in mind when designing the incoming waves.
Adding to this is placement of towers and managing upgrading vs. building new towers.
Knowing what each tower does, and how it changes with each upgrade is essential for success
in difficult levels. Experimentation is an important part of gameplay in tower defense games.
Through experience and testing the player knows all towers and upgrades and is able to predict
how they will work in the map.
Because the amount of health increases from wave to wave, the player must be investing in
more tower power every wave, so that the amount of tower power can always deal with the
increase of health. If the player keeps the gold he won’t transform it into tower power and in a
matter of a couple of waves the tower power will not be able to deal with the increase in creep
health. Some creeps (if not all) will reach the exit.
26
6. Strategy
6.1. What is a strategy
6.1.1. Definition of Strategy:
Dr. Vladimir Kvint defines strategy as "a system of finding, formulating, and developing a
doctrine that will ensure long-term success if followed faithfully." (Kvint, Vladimir & Kvint 2009)
In my words and modified to the tower defense context: A set of instructions for placing and
upgrading towers.
6.1.2. Dominant strategy.
Simply put a dominant strategy is a strategy that always wins. Having a set of instructions that
no matter what the opponent does or what the conditions of the level/map are, always wins.
(Gametheory.net n.d.)
The problem with having a dominant strategy is that it transforms the game into a mechanical
action, where there is no analysis, no thought on cause - effect, no puzzle solving. When a
player finds a dominant strategy he will continue using that strategy, this in turn makes the
game not challenging, predictable and boring.
6.1.3. Tower defense dominant strategy.
Pairing a slow tower with an area of effect tower and placing a high damage single target tower
further along the path to take care of leaked creeps. The slow tower makes all creeps slow
down so that the area of effect tower has more time to weaken and kill all creeps, in case a
creep gets by alive and goes outside the range of the area damage tower, there is a high
damage single target tower to take care of it. Towers will be upgraded before buying new towers
and it must be done so as soon as the budget allows. This in order to avoid that the increase in
creep health catches up with the player’s tower power.
This example works for games that are balanced but where there are no strength or
weaknesses by damage type, like we can see in Kingdom Rush Frontiers, where there is armor
resistance and magic resistance and flying creeps.
6.2. Intransitivity. Rock, Paper, Scissors.
Intransitivity is the opposite of a dominant strategy.
We can take the game Rock, Papers Scissors (RPS) as an example. There are three options in
total, each wins against one and loses against the other, no matter what option the player
chooses to play, he may win or lose depending on what the opponent plays. There is no
certainty. No dominance.
27
6.3. Examples of using the balance elements as strategy.
Following the thought that tower defense games are puzzles, here I will show examples of how
each balance element can be used as a solution for a specific problem.
6.3.1. Range.
Increasing range allows the tower to attack a path that is just outside of range without the range
upgrade. This in turn allows the tower to attack creeps in more places along the path so that
creeps have to gain much more ground to get outside the range of a tower.
Situation (Fig. 9): In mathematical terms, if in a grid based map, a tower reaches 9 nodes, the
wave of creeps consists of a total of 30 creeps are spawned one every second, they walk at a
rate of one node per second and have 10 health points. The fire rate of the tower is 1.5 seconds
and the damage is 11. So every time the tower fires, it kills the creep, but by the time it’s ready
to fire again, the position of the next creep will be half a node further along the path. Having 9
nodes of action for the tower, it can kill 18 creeps before the first creep is leaked. After that the
tower will kill the next creep in range twice before leaking again, this is repeated until the wave
is done or the game is lost. This wave has 30 creeps, the first 18 are killed leaving 12 more to
kill if we divide this number by three we get the number of creeps leaked, 4.
Increasing damage does not change the number of leaked creeps because the initial damage is
already enough to kill a creep in one shot.
Looking at the map, the position of the tower and the shape of the path, we can see that by
increasing the range we can add 7 more nodes to be in range of the tower taking the nodes in
range from 9 to 16, which will allow the tower to kill all 30 creeps without leaking any of them.
Fig. 9. Hypothetical situation 1.
28
6.3.2. Damage.
In case the damage a tower is inflicting is not enough to kill a creep fast enough to stop them
from gaining ground, increasing damage may be a viable solution.
Situation (Fig. 10): We have a map where a tower has 9 nodes within reach, there is an
incoming wave of 30 creeps total, with 15 health each one, spawning every second and a speed
of 1 node per second. The tower has 11 damage power and shoots once per second, so it takes
the tower 2 shots and 2 seconds to kill one creep. The next creep to target will start already one
node further in along the path in comparison to the previous creep. 9 nodes in range of the
tower will only allow the tower to kill 8 creeps, the ninth creep will receive one shot and by the
time the tower is going to fire again it will be out of reach, the same thing happens with every
creep after that. 22 of the 30 creeps are leaked.
Increasing the range will make 2 more nodes come into range, with 11 nodes in range instead
of the initial 9 the tower will now be able to kill 10 creeps, leaking the remaining 20.
Increasing the fire rate will only increase it to fire once every 0.8 seconds, which will reduce the
time it takes the tower to kill one creep from 2 seconds, to 1.6 seconds, making the next creep
gain 0.6 nodes instead of one, but it is still gaining ground. Increasing the fire rate will allow the
tower to kill 15 creeps, leaking the remaining 15.
If instead we increase the damage inflicted by the tower, it will now deal 16 damage every time
it fires, allowing the tower to kill a creep in one shot. The next creep in range will not gain any
ground and will be killed at the same position along the path as the previous one. All creeps are
killed without leaking.
Fig. 10. Hypothetical situation 2.
29
6.3.3. Fire rate.
Increasing the fire rate of a tower helps with fast creeps that move fast. Even when the tower
has enough damage per shot to kill the creep, if creeps move fast enough they will steadily gain
ground. Increasing the fire rate allows the tower to counter the fast speed of these creeps by
killing them faster, before they gain enough ground to go outside of reach of the tower.
Situation (Fig. 11): A tower has 5 nodes in range, deals 11 damage and fires once per second.
Incoming wave is 30 creeps strong spawning one every second, each with 10 health points
moving at 1.3 nodes per second. The tower will kill a creep every time it fires, the next targeted
creep will be 0.3 nodes further along the path. The tower will kill 16 creeps but will not be able to
kill the 17th. Creep 18th will be one node away from going out of reach when it is killed and the
next 3 creeps, up to the 21st will also be inside range of the tower but creep 22nd will be outside
of range when the tower is ready to fire again. And this is repeated once more, allowing creep
26st to pass by unharmed. 3 creeps are leaked.
Increasing the range will bring 2 more nodes in range. 22 creeps are killed before the 23rd is
leaked, again, next four creeps are killed by the tower and 27th is leaked, the three remaining
are killed. 2 creeps are leaked.
Increasing damage does not change the number of leaked creeps because the initial damage is
already enough to kill a creep in one shot.
Increasing firing rate will allow the tower to shoot faster, thus having the desired effect of
shooting before the creep can gain much ground if any at all. If the fire rate is increased from
once per second to once every 0.8 seconds. Creeps have a speed of 1.3 nodes per second, in
0.8 seconds they travel 1.04 nodes, so by the time the creep gains 0.04 nodes of distance along
the path it is killed. This eliminates almost completely the ground creeps gain. When the last
creep of the wave is killed they will have gained 1.2 nodes. No creep is leaked.
30
Fig. 11. Hypothetical situation 3.
6.3.4. Damage conditions.
Changing the damage conditions is very useful when a group of creeps is very close together. A
common strategy when mazing is available is funneling all creeps to an area where they will be
slowed, and at the same time in range of the area damage tower, so that by first slowing the
creeps they will come closer together, and then the area damage tower will deal damage to all
creeps that are inside the splash area damage.
Situation (Fig. 12): A wave of 30 creeps are all very close to one another, the towers has 9
nodes in range, fires once per second dealing 11 damage, The creeps all have 10 health points
and move at 1 node per second. The tower only gets to kill 9 creeps before they all go out of
range, leaking 21 creeps.
Increasing the damage changes nothing, as the current damage is more than the health of one
creep.
Increasing firing speed will allow the tower to kill a couple more creeps before they go out of
range, but still more than half will be leaked.
Increasing range will add three more nodes to be in range, which will allow the tower to kill three
more creeps, but still 18 creeps will be leaked.
Changing the damage conditions will allow the tower to hit all creeps inside the splash area
damage. All creeps of the wave are very close together, by hitting one of them, all the rest of the
creeps on the wave will be hit as well. Even if the damage amount is decreased when the
damage conditions is changed, the tower will easily deal with the group of creeps before they go
out of range.
31
Fig. 12. Hypothetical situation 4.
6.4. Adding types as a new balance element
Looking back at the creeps on Kingdom Rush Frontiers, we can see that some of them have
magical resistance and some of the have armor, measured as low, medium and high. The effect
this has on gameplay is that no one tower is effective against al creeps. An archer tower is
effective against creeps with no armor or magical resistance, but not effective against armored
units. Likewise, the mage tower is effective against armored units, but not so against magic
resistant creeps. In a way both this tower complement each other, they are both ranged towers,
but each has a type, an affinity. Furthermore there are flying units, Archers and mages can
attack flying units but artillery towers attack only ground units.
In Fieldrunners2 we can also see something similar, there are some creeps that have suits on,
depending on the suit the creep has a resistance to a type of attack. There is a fireproof suit, a
hazmat suit and an antifreeze suit. If the players strategy relies heavily on fire towers, when the
fireproof creeps come along, they will go by without being affected by the fire towers. The same
goes for the toxic tower which the hazmat unit counters. Fieldrunners2 also has flying units, not
all towers attack flying units.
The player must look at the map he is playing, know what type of creeps will be in the coming
waves and make a strategy accordingly.
32
7. Research
In order to have a bigger reach and be open for people from around the world, an online
website was created to allow testers to play the game and answer questions. The site can be
browsed in four different languages and all questions answered are automatically stored in a
database.
The report can be seen online at: http://www.vrolfak.com/rakshati/user_test/report/
7.1. Prototypes
Four game prototypes were made. All prototypes are tower defense games but each has
different mechanics in play. The mechanics for each prototype were chosen based on the
analyzed games.
7.1.1. Prototype 1
This prototype (Fig. 13) is a simple tower defense game where the user only needs to click
predefined building spots to build the tower and then upgrade them. There are only three
different towers to choose from, each with a different price. They represent a cheap tower, a
middle price tower and an expensive tower. Towers have 4 levels, building them would be the
first level and then they can be upgraded three times. The towers have small blue squares that
represent what level the tower is at. A tower that has not been upgraded has no squares, a fully
upgraded tower will have three blue squares.
Fig. 13. Prototype 1.
33
7.1.2. Prototype 2
This prototype (Fig. 14) adds 3 basic tower elements as options when upgrading so that when
the player clicks a tower to get the tower options and range info, they can now select between
Damage, Fire speed and Range. Each option increases only the mentioned stat. So that when
the player chooses to increase the Firing speed, the tower will shoot more times per second, but
the damage and range will be unchanged. And so on with each upgrade option. Each option
has 3 levels so after building the tower, each option can be upgraded 3 times. All three options
can be upgraded independently and to the max level. The blue squares that represented the
upgraded level of the tower now represents only damage stat, the range can be seen when the
tower is selected, and the Firing speed can be seen by the amount of bullets the tower fires.
Fig. 14. Prototype 2.
7.1.3. Prototype 3
This prototype (Fig. 15) removes the upgrading options and goes back to having just one
upgrade option. But here we add a grid for placing towers, instead of clicking predefined
building spots, the user can drag the towers from a menu and place them on the grid. Towers
cannot be placed on the path.
34
Fig. 15. Prototype 3.
7.1.4. Prototype 4
In this prototype (Fig. 16) I remove the menu from the interface and like the previous prototype,
there are no predefined building spots. To build a tower the user clicks the grids square where
he wants to build. There is no predefined path, the creeps search for the fastest route to the
exit, if a tower is built on the path the path changes to go around the tower. This is called
mazing. The path cannot be blocked. Towers have two upgrade levels after being built, which
increase all stats. No upgrade options like in the second prototype.
Fig. 16. Prototype 4.
35
7.2. Questions
The goal of these questions is to determine how people perceive playing different types of tower
defense games, to see if players can easily distinguish between different mechanics inside the
genre, to have an idea of how players react to different mechanics and also to know which
mechanics might be more enjoyable. Also to see if players in general have a desire to upgrade
towers and while upgrading them, to see if they like having different options instead of having
just the one button to upgrade.
There is a total of 22 questions asked through 5 pages. First three general questions after
registration and then one page of questions after each prototype.
7.2.1. General questions
I ask these questions so that if necessary, I can filter the results for different people, for men
and women, for people who are gamers and people who are not, and people who already know
hot to play TD games and those who have not played them yet.
1. Are you familiar with tower defense games?
1. Yes.
2. No.
3. I've heard about them.
4. I love TD games.
2. Do you play games a lot?
1. Hardcore!
2. Casual gamer.
3. I don't play that much
4. NO!!! Games are a waste of time!
3. I am a
1. Man
2. Woman
7.2.2. Prototype 1
These questions are asked after playing the first prototype.
1. Were the instructions clear?
1. Yes
2. No
3. What instructions?
4. It took me a while to see them.
Each prototype has instructions below it. Because each prototype has a different mechanic then
each has instructions. They are not repeated, only new instructions are shown. This question is
asked again after each prototype.
36
2. Which tower did you use the most?
1. Gray
2. Red
3. Blue
4. I used them all equally
5. I don't remember.
Another question asked after playing all each prototype. This questions helps to understand
how players test and try different towers. It is here to see the progression of tower choice along
the test, thinking that it would be the same as in an actual game. Do players play always with
the first one they choose? or do they try different towers in different prototypes? or do they
always try combinations? Do they test until they find the best one?
3. Would you say the game is intuitive?
1. Yes
2. No
3. I had no idea what I had to do.
Tower defense games are popular and present in all game environments, specially in mobile
devices and browser games. Is this game intuitive? can something be done to make it more
intuitive?
4. Did you upgrade the towers?
1. Yes, but only one level.
2. Yes, to its maximum!
3. Hhmm... upgrade?
4. No, building towers to make a maze was more important.
This question serves two purposes. It helps to find out if players actually upgrade towers and
how much they upgrade them.
7.2.3. Prototype 2
This are the questions asked after playing the second prototype.
1. Did you notice the difference between the two games?
1. Yes
2. No
3. There was a difference?
4. I don't remember the first game any more.
These questions are here to find out if players can recognize the difference (in mechanics)
between two games, even if they are in the same genre.
37
2. What tower did you use the most this time?
1. Gray
2. Red
3. Blue
4. I used them all equally
5. I don't remember.
3. Did you upgrade the towers?
1. Yes, but only one level.
2. Yes, to its maximum!
3. What upgrades?
4. No.
4. Do you prefer having options for upgrading the towers?
1. Yes.
2. No.
3. I'm not sure.
4. I don't understand this question.
After playing the first two prototypes the player has experienced having only one button to
upgrade a tower and having options for upgrading different balance elements. Asking this
question helps identify if players have a good adoption for this mechanic.
7.2.4. Prototype 3
These are the questions asked after playing the third prototype.
1. Did you notice the difference between the second and third games
1. Yes
2. No
3. There was a difference?
4. I don't remember the second game any more.
2. What do you prefer? Upgrading a tower or building a new one?
1. Building more towers.
2. Upgrading towers.
3. I don't see the difference.
This questions helps get a better idea of player behaviour. Do all players think alike? Is there a
difference in behaviour between hardcore, casual games and non gamers?
3. Did you place a tower on the path?
1. Yes.
2. No.
38
3. What path?
This question is to find if players by nature try to place towers on the path when having the
option of positioning towers by dragging them from a menu. Knowing how players act will help
when designing the learning curve, hints and tutorials for the game. It is related to how intuitive
the game is.
4. Would you like it better if the last game also had options when upgrading towers?
1. Yes.
2. No.
3. Hhmm... I'm not sure.
4. I don't really care.
The first time this question is asked the player had just been presented with this new mechanic.
But now the mechanic is no longer in place, asking this question again allows the tester to think
about this again and reply from a more experienced position. Did he give the same reply to both
questions? Did he say yes before because it was a new thing? Does he still think having options
is a good thing? Did any player say no at first, but say yes when the options were not there any
more?
7.2.5. Prototype 4
These are the questions asked after playing the fourth prototype.
1. Is the difference between the last two games clear?
1. Yes.
2. No.
3. I'm not sure.
1. Did you place a tower on the path on the last game?
1. Yes.
2. No.
3. No, because it wasn't allowed in the previous game.
4. Yes, somehow I knew it was allowed.
The fourth prototype is the only prototype that allows mazing as a strategy.
2. What tower did you use the most this time?
1. Gray
2. Red
3. Blue
4. I used them all equally
5. I don't remember.
3. Did you upgrade the towers in the last game?
39
1. Yes.
2. No.
This is another question that helps study player behaviour. Having the option of mazing as a
strategy only in this prototype, did players go for mazing? Did they practice the same strategy of
upgrading towers as much as possible as in previous prototypes?
4. What image better resembles your last attempt?
1. No mazing at all.
2. Path modified.
3. Expert maxing.
Another question to study player behaviour, how do players from the different groups (hardcore,
casual gamers, non gamers) play? Is there an evident difference in play styles?
7.2.6. General questions 2
After the fourth prototype there is no more play test, so in order to follow the previous flow of the
site, questions - prototype - questions, the questions were added to the last questions page.
1. Which interface did you like better?
1. Prototype 1.
2. Prototype 2.
3. Prototype 3.
4. Prototype 4.
5. No preference/All the same.
Another question to study what player behaviour. What interface was most liked? Will making a
game with the most popular interface give that game a better chance of success as opposed to
implementing the least liked interface?
2. Are you enjoying these games?
1. Yes, a lot!
2. It’s ok.
3. It started good but now I’m bored.
4. Not at first but now it’s cool.
5. No.
Knowing the fun factor of the game is very important.
7.3. Results gathered
There were a total of 127 men and women tested. Social media (Facebook, twitter) was used to
spread the word about the survey and ask people to participate. Ages vary from 10 to 61 years
old.
40
7.3.1. General questions
1. Are you familiar with tower defense games?
2. Do you play games a lot?
A little over half of testers, 54.7%, are gamers.
3. I am a
41
7.3.2. Questions for prototype 1
1. Were the instructions clear?
The instructions were short, concise, sentences.
Only half of testers, 53%, found the instructions clear.
2. Which tower did you use the most?
Blue tower was used almost 50% more than the next most used tower.
3. Would you say the game is intuitive?
42
Even though only half of testers found the instructions clear, 66.7% found the game intuitive.
More people knew what to do from context than from instructions. Tower defense games are
simple in premise. We can take from this that there is no real need to be overly explanatory.
Instructions need to be a little better, but in the end, players will find their way without much
difficulty.
4. Did you upgrade the towers?
Over 70% of testers made use of upgrading.
7.3.3. Questions for prototype 2
1. Did you notice the difference between the two games?
43
The number of testers that can see a difference in mechanics is a majority.
2. What tower did you use the most this time?
Blue tower is still the most used, but the difference with the second most used tower is not as
much as in prototype one.
3. Did you upgrade the towers?
Adding all testers who upgraded towers we get a bit majority of 80.3% who make use of
upgrading. Up 10% more testers from the 70% that upgraded in prototype one.
44
4. Do you prefer having options for upgrading the towers?
Three quarters of testers like the fact they could choose when upgrading.
7.3.4. Questions for prototype 3
This are the questions asked after playing the third prototype.
1. Did you notice the difference between the second and third games?
A big majority of 93.6% of testers can see the difference between the previous two games. Up
from the 83%
2. What do you prefer? Upgrading a tower or building a new one?
45
There is not much of a difference between those who prefer to upgrade towers and those who
prefer to build more towers.
3. Did you place a tower on the path?
4. Would you like it better if the last game also had options when upgrading towers?
A big 68% are sure they like having options as opposed to only having one upgrade button.
7.3.5. Questions for prototype 4
1. Is the difference between the last two games clear?
46
Again the group of testers who see the difference in mechanics is a big majority.
2. Did you place a tower on the path on the last game?
3. What tower did you use the most this time?
Blue still in the lead, though the others are not as behind as on the previous games.
4. Did you upgrade the towers in the last game?
47
Upgrading went down in the last prototype. Seeing the increase in upgrading in the first three
prototypes I am leaning to believe that the decrease in upgrades here given to the fact that the
mazing mechanic was introduced and to create a good maze a big number of towers is
necessary, motivating players to build more towers instead of upgrading them.
5. What image better resembles your last attempt?
58% of testers show a strategy behind the placement of towers.
Looking at question two of this set we can compare data. 64.4% built towers on the path, and
here adding the users who made mazes (41.8%) or half mazes (21.5%) we get 63.3%. This
corroborates that the data is right.
7.3.6. General questions 2
1. Which interface did you like better?
48
The majority of testers are between two interfaces, prototype 2 and 3. The difference in
mechanic between these two prototypes and the first and fourth is upgrade options and a grid
based level for placing towers around a predefined path
Choosing prototype 2 testers show a preference for having upgrade options.
Choosing prototype 3 testers show a preference for choosing tower placement. Which in turn is
having more options.
2. Are you enjoying these games?
Over 90% of testers had fun at some point of the game, with over 65% having fun either all of
the game or most of it. Less than 10% did not find the games any fun.
Knowing that 43.8% knew nothing about TD games before the test and an extra 15.4% had only
heard about them, almost 50% testers were introduced to TD games, of this, the 90% of this,
(45%) of all testers were new players that had fun. There is a lot of room for TD games market
growth.
7.4. Analyzing the data
Relevant comments are written under most questions but I will summarize my conclusions here.
First thing I see in the results is that people can easily notice the differences in mechanics from
different tower defense games. Even though it is the same genre playing two different tower
49
defense games can give the player different experiences. Players can easily distinguish one
tower defense game from the next based on the mechanics. This can explain why some TD
games are highly successful and others are not. Even a small mechanic change can have a
positive or negative impact on the gameplay of a game.
Over 70% of the players upgraded towers at least once in the first prototypes, increasing from
one to the next, But only 50% upgraded on the last one. This makes sense because after each
prototype they are asked if they upgraded a tower and how much, so it is understandable that
they upgrade more towers in the second and third prototypes. Also because of learning curve,
players might not do everything on the first game but then do more on the next ones. Keeping in
mind that the first two prototypes only have a few building spots for building towers, after all
spots are built on, the only option for increasing tower power is upgrading the towers already
built. In the last prototype, it also makes sense that the number of people who upgraded towers
goes down, because one of the main strategies for that particular prototype is mazing, which
involves building towers to make a path for the creeps. This requires building more towers as
opposed to upgrading them.
75.5% liked having options on the second prototype and 67.6% answered that they would like
the 3rd game better if it had upgrade options. From this we can conclude that most people like
having options when upgrading, as opposed to just having one button upgrades for towers.
The difference between having options and not having them, when upgrading, is that having
one button for upgrading a tower is like a closed box, you know that the tower will be more
powerful, but you do not know what that implies, after analyzing tower defense game
mechanics, we can see that a tower can become more powerful in different ways, and just
having one button for upgrading a tower does not give the player much information on what is
going on inside the tower. While having options is an open box where we can see inside the
tower and we know what is happening, we can incorporate this freedom into our strategy, not
mentioning that the number of strategies goes up and now there are more things to play with.
Having options for upgrading towers gives the player the ability to choose how a tower becomes
stronger.
Interface preference shows that overall the majority of players prefer having options, be it
upgrade options for the towers or having a grid based map where there are so many more
options for placing towers.
Knowing that the most expensive tower should be the most powerful one it is not surprising that
the blue tower was the most used overall.
Being as familiar as I am with TD games, It is surprising for me to see the amount of people who
try to place towers on the path. Even after trying once and not being able to do it, they try again
in the next game. This makes me think of GemCraft where the player can build permanent traps
on the path. It also makes my think of Fieldrunners2 and kingdom Rush Frontiers, but in these
cases what the player places on the path are consumable items like bombs or a special powers
like fire.
50
8. Conclusion
Conclusions are included in each chapter but for organization and ease of use I will summarize
them here again.
What is a tower defense game?
Short version:
“In a tower defense game the player attempts to stop enemies from moving across a map by
placing static defensive units at strategic locations.”
Long version:
“A tower defense game is a strategy game where the player positions fixed units (towers) that
will automatically attack enemy units (creeps) in order to kill them before they traverse the map.
The path creeps take may be fixed or dynamic, changing by placement of towers. The strength
of the creeps grows with every wave so the difficulty is ever increasing. Towers are built using
money, which is acquired by killing creeps. A match may have winning conditions, like surviving
X number of waves, or it may be endless and last until the player loses by not killing the creeps
before they cross the map. Towers can be upgraded to make them stronger so they can deal
with stronger creeps.”
Keep in mind that the game designer has the last word and decides what mechanics if any will
be implemented in the game.
What are the balance elements in tower defense games?
This questions is here to help understand how a TD game works, so that later we can give the
players that freedom of choice. But this freedom of choice only pertains to the towers. Here are
the balance elements for the towers:
Basic tower properties:
- Damage: The amount of health a creep loses when hit by a tower.
- Firing speed: Amount of time it takes a tower to reload and be ready to fire again.
- Range: Maximum distance that a tower can target a creep.
- Damage conditions: Whether the tower attacks one or several creeps at the same time.
Single damage, splash damage or damage all around.
Additional tower properties:
51
- Specific damage type: Like magic, poison or plasma.
- Combo damage: When certain towers are built next to each other, an extra damage
occurs.
For the complete list of balance elements refer back to chapter 4.
How to make tower upgrading non-linear to allow adapting to different scenarios?
We can use the balance elements as individual upgradable properties for the towers.
How to balance the tower elements to avoid a dominant strategy?
As shown in chapter 6.3 where I write about adding damage types as an extra balance element.
By adding damage types we put the players in a situation where they must always keep in mind
the strength and weaknesses of their defense. If their defense is based on fire, what happens
when a fire resistant creep comes along? By adding this extra element of type resistance to the
creeps, the player cannot just follow an order of actions to win; he must look at the incoming
creeps and adapt his defense accordingly.
How is the emotional effect changed from current TD games?
The overall experience of the game is still the same, the emotional effect is not changed much,
by being a tower defense game the experience will be similar to other tower defense games.
The difference lies in the variety of challenges that can be presented to the player and the
amount of personality that can be shown in the strategy that each player implements.
From personal experience, when playing a game that gives me choices and exploration, I also
find a great intellectual challenge and therefore a great sense of victory when the game is won.
By having the possibility of implementing and testing different defense strategies I can expect
that the fun factor and the satisfaction will be even greater. This is reinforced by the 90% of
testers that had fun while playing the prototypes.
Level design plays a very big role in giving players different map experiences that use different
strategies for winning.
“How does giving players the freedom to design their own towers, affect the
mechanics of tower defense games?”
The mechanics are not affected by giving players upgrade options. The delicate issue is
keeping the economy in check, making sure that the price of upgrading strengthens the
economy of the game and the damage vs. cost ratio is respected so that the balance of the
game is not broken. If the cost of upgrades is not dynamic and adapts to different scenarios the
economy and balance of the game will be broken and in so doing will break the game. If there is
no challenge or the game is impossible, it will not be fun.
Now let us see some numbers!
52
In the next example the damage information is per second. I’ll use round numbers to make the
math easier to understand. And the damage vs. cost ratio is one; meaning that for each gold we
invest in upgrades we should increase the tower power by one damage.
I want to upgrade a towers damage and firing speed, without upgrades the tower shoots twice
per second and deals 20 damage, which means it deals 10 damage each time it fires. The firing
speed upgrade will make it fire three times per second instead of two, 10 damage extra so the
cost of this upgrade should be 10. The damage upgrade will increase the damage from 20 to
40, that is 20 extra damage so the damage upgrade should cost 20.
If we do upgrade firing speed first, the tower now shoots three times per second dealing 30
damage. The damage upgrade is the same, it has the same effect now that it would have had
before, meaning that it doubles the damage inflicted. Increase in damage is 10, multiplied by the
number of times the tower shoots per second gives us 10 * 3 for a result of 30. The damage
increase will be 30 extra damage taking the tower from 30 to 60 damage per second.
This all happens to respect the economy of the game, to keep the damage vs. cost ratio
constant, to maintain the value of gold always at the same level. If this kind of calculations is not
made and the upgrades have a fixed price, the ratio would not be constant, giving the player
more damage for a lower price and breaking the economy of the game.
As a surprise I found that TD games are more puzzles than strategy games. I can say that I
agree with this statement, the strategy aspect of the game works like solutions to problems,
what tower is best against certain creeps? And so on. And at the same time that is also what a
good strategy is, managing the current resources as best as possible. After playing so many
different tower defense games it is a big surprise to find out that this genre, which I though I was
very familiar with, still had more surprises left.
53
9. Bibliography
Apple, 2014a. CarrotFantasy on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/carrotfantasy/id534453594?mt=8 [Accessed August 13,
2014].
Apple, 2014b. Fieldrunners 2 on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/fieldrunners-2/id527358348 [Accessed August 13, 2014].
Apple, 2011. GemCraft on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/gemcraft/id418510736?mt=8 [Accessed August 13, 2014].
Apple, 2014c. Kingdom Rush Frontiers HD on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/kingdom-rush-frontiers-hd/id598581619?mt=8 [Accessed
August 13, 2014].
Apple, 2014d. Pirate Legends TD on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/pirate-legends-td/id626884809?mt=8 [Accessed August
13, 2014].
Apple, 2013. Royal Defense: Invisible Threat on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/royal-defense-invisible-threat/id590224579?mt=8
[Accessed August 13, 2014].
Apple, 2012. TapDefense on the App Store on iTunes. Available at:
https://itunes.apple.com/us/app/tapdefense/id297558390 [Accessed August 13, 2014].
Avery, P. et al., 2011. Computational intelligence and tower defence games. 2011 IEEE
Congress of Evolutionary Computation (CEC), pp.1084–1091. Available at:
http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=5949738.
Gametheory.net, Dominant Strategy - Game Theory .net. Available at:
http://www.gametheory.net/dictionary/DominantStrategy.html [Accessed August 11, 2014].
Kongregate, Play Desktop Tower Defense on Kongregate. Available at:
http://www.kongregate.com/games/preecep/desktop-tower-defense [Accessed August 8,
2014].
Kvint, Vladimir & Kvint, V., 2009. The Global Emerging Market: Strategic Management and
Economics’', Routeledge. Available at:
http://books.google.com/books?id=mb5n8O9y6YIC&printsec=frontcover&dq=the+global+e
merging+market&hl=en&sa=X&ei=12DLUprvKfLKsATS54GIDQ&ved=0CEgQ6AEwAQ#v=
onepage&q=the global emerging market&f=false [Accessed August 5, 2014].
54
Portnow, J., Gamasutra - Opinion: Redefining Casual For The Hardcore. Available at:
http://www.gamasutra.com/php-bin/news_index.php?story=23249 [Accessed August 11,
2014].
Wetzel, B. et al., 2012. If Not Now, Where? Time and Space Equivalency in Strategy Games.
Eighth Artificial Intelligence and Interactive Digital Entertainment Conference, pp.81–86.
Available at: http://www.aaai.org/ocs/index.php/AIIDE/AIIDE12/paper/view/5456/5701.
Wikipedia, Wikipedia, the free encyclopedia. Available at:
http://en.wikipedia.org/wiki/Tower_defense [Accessed August 13, 2014].
55