Our first analysis will be of Onslaught Tower Defence. Onslaught is one of the many, many, tower defense games on the web. If you're not familiar with the genre from its PC roots, you may recognize console equivalents like PixelJunk Monsters or Ninja Town. A couple minutes playing Onslaught should get you familiarized pretty quickly though, so try it out (just not on Hard difficulty...)
Flash games can be great design exercises in balancing, because they are generally simplistic and have fewer player choices to wade through. Whereas a complicated PC game like Civilization II can get by with imperfect balance simply by having more than enough choices, in a flash game you can usually tell within a few games what the actual "valid" choices are. Onslaught proved to be a perfect example.
Starting on "Hard" difficulty, a cursory view revealed 4 kinds of turrets to start with, one of which was too expensive to build the first round. Picking the most expensive I could afford ($20 for a missile launcher), I placed one and watched it one-shot every enemy in the first four waves. Alright, seemed like a solid first choice. After several waves, I had many rocket launchers down and discovered individual turrets could also be upgraded at a relatively high cost ($30 - $50 to start) for improved damage, rate of fire, and range.
Usually in tower defense games, when upgrades for turrets are available, upgrading them is mandatory to beat the game.
I had two apparent choices at this point:
- Upgrade one turret to the exclusion of all others
- Upgrade a number of turrets evenly
This is a classic balancing problem. In tower defense, the core challenge is a matter of "how much damage can my turrets deal" vs. "how many hitpoints do all the enemies in a wave have". In order to perform well, I want to make the choice between the two above that will maximize my turrets' damage for the money I spend on them. To make the player's decision interesting, each choice should be roughly equal in power, and have a strength or weakness to differentiate it from the other choice.
Unfortunately, Onslaught only had one valid choice: upgrade one tower. By the 4th upgrade, the amount of damage the tower deals begins to jump dramatically: 1950 damage for $720 total upgrade cost (2.7:1 damage to cost, with a basic turret giving 50 damage for $20; a 2.5:1 ratio), and it only got more extreme from there. You can see in the screenshot the "damage" bar of the turret goes up exponentially. Basic turrets deal damage in the 10s range. A fully-upgraded turret deals damage in the area of 100,000 points. Assuming all of the towers had damage that scaled exponentially, I now knew the only valid strategy in this game: build one tower, upgrade it to the max, build your next tower, upgrade it to the max, etc.
My valid choices in this game have probably just been cut dramatically. And after building several turrets and upgrading each of them evenly, I eventually was overcome by the horde of printer icons and lost my game - my first impression holding true, that upgrading multiple turrets evenly was a completely invalid strategy for Onslaught (on Hard).
Here's how I might balance this to give the player valid choices:
Missile turret - cost to build: $20
Damage: 50
Upgrade level 1 - cost: $20
Damage: 95
Upgrade level 2 - cost: $40
Damage: 175
As you can see, for $40, a player could build an upgraded level 1 missile turret, or two basic missile turrets. For $80, the player could build one upgraded level 2 missile turret, or two upgraded level 1 missile turrets, or four basic missile turrets.
$40:
Upgraded level 1 missile turret: 95 damage
Two basic missile turrets: 100 damage
$80:
Upgraded level 2: 175 damage
Two upgraded level 1s: 190 damage
Four basic missile turrets: 200 damage
Looking at just these numbers, our first reaction should be: it's better to not upgrade your turrets! If the player wanted to get the most bang for their buck, they would build as many missile turrets as possible, and never upgrade them. Upgrading turrets would be a weaker (possibly invalid) choice for the player.
...that is, unless we make a wave with monsters something like this:
Wave 42 - 2x Armored Triceratops
Hit Points: 20000
Damage Reduction: 25
This monster has something that I'll call damage reduction: for every shot he receives, he takes 25 less damage. This makes our basic turrets far less effective than the upgraded turrets, suddenly:
Missile turret - cost to build: $20
Damage vs. Triceratops: 25
Upgrade level 1 - cost: $20
Damage vs. Triceratpos: 70
Upgrade level 2 - cost: $40
Damage vs. Triceratops: 150
If Triceratops type monsters came sprinkled among the regular waves, the player would be faced with two valid choices and an interesting dilemma: do I want to build lots of small turrets that will be more effective against unarmored targets, or do I want to build a bunch of large turrets that will be more effective against monsters with damage reduction?
Adding strengths and weaknesses to similarly-powerful decisions is the way to make valid player choices. Various tower defense games have other methods of balancing towers against each other, with abilities like slowing enemies and dealing damage to multiple enemies in an area.
Commentsercize
Here's an exercise for you. Answer in the comments: What are other things you could put on turrets to make them interesting against each other in a Tower Defense game?
I've already listed two: turrets that slow and turrets that deal damage to multiple enemies in an area.
Thesis Work
If you have more time than just a comment and want to practice design, here are some exercises:
- Find some other invalid (or simply less-valid) choices in Onslaught Tower Defense.
- Describe why each is an invalid player choice.
- List how you might personally fix each of them.
- Make a design for a tower defense game, focusing on the choices the player has in spending their money.
- Show in the design how all the choices the player has in building and upgrading their turrets are "perfectly" balanced.
- Utilize new and interesting ways you make different turrets/upgrades similarly powerful, but meaningfully separate choices.
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