Onslaught Tower Defence - Part 2

ONE VALID SOLUTION

Last week we talked about balancing upgrades vs. more units in the context of the flash game Onslaught Tower Defence. In our first game, we realized on Hard difficulty, we couldn't survive by just making a lot of turrets and upgrading them a couple levels each. The turret statistics were balanced heavily towards upgrading a single turret to get the most damage for your money; a classic design decision that frequently removes a lot of player choices that might have added to the strategic depth of a game.

So starting a new game on Hard, this time I built a single rocket turret and did nothing but upgrade it as soon as I had the money. Things were going nice until about wave 22, when the HP of the monsters caught up with the damage on my turret anyway.

Well this was odd. The numbers didn't end up supporting either method. Upgrade a bunch of turrets, you die. Upgrade one turret, you die. I tried a couple of combinations: using a slowing turret along with a main damage-dealer; trying another line of turret to upgrade; messing with the rate of fire and range upgrades. But after a couple combinations, I started to become suspicious. The numbers were right there in front of me - the setup was simple enough that obviously damage was by far the most effective key in the puzzle - but that key didn't seem to be working.

So I gave up on the puzzle (I hate puzzle games) and hopped on the forums. And there I found the answer - the most eggregious of balancing problems - there was only one specific strategy that would allow you to survive Hard mode.

Turns out the laser turret can be strung together in chains to multiply their damage. This laser chain was the only build combination with a damage output high enough to overcome Onslaught's early game to the point you could get turret combos going to defeat the rest of the 400 waves.

So in the end, Onslaught Tower Defence (at least on Hard mode), only had one viable strategy. All other strategies the game presented to the player were actually just eventual failure conditions. From a strategic sense, the game might as well be something like:

"Would you like to:

1. Build a laser chain.
2. Build a lot of pellet turrets.
3. Build a lot of missile turrets.
4. Try a mixture of turrets.
5. Build one missile turret, and upgrade it fully before starting a second turret.
6. Etc.
7. etc.
8. etc.

If player-did-not-choose-option-1: Game Over

Try again?"

In essence making it a puzzle game, not a strategy game.

The point of balanced design, and making multiple valid choices available to your players, is to add depth to an existing structure. Lots of games get by fine without any depth. Onslaught Tower Defense is a fun romp in annihilating waves and waves of little icons in various ways with instant-kill, super combos once you upgrade a couple turrets. The fun isn't in the deeply strategic ways they are employed to give you an advantage; the fun is build yet another invincible combo and watching the nifty way it obliterates creeps.

Other games rely on strategic depth completely. Chess, checkers, Othello, and others all have very little in the way of eye-candy or theme (or especially colorful, hardware-accelerated graphics) going for them to make them interesting beyond their balanced design. A small group of games are strong on their own, but are made immortal by their engaging depth of play: StarCraft and Street Fighter are always easy examples.

The fix for adding depth to Onslaught on Hard is simple: make more turrets or turret combinations able to equal the effectiveness of a laser chain (or even just enough power to complete Hard mode). The more options that are valid, the more time the player might invest into trying each of those options. We'll talk more next week about that: what I like to think of as a critical mass of player options.

There are many ways to make something equally "effective" to something else, even for something as straightforward as a Tower Defense game. The obvious choice is to equalize damage, but there are other ways as well. In Tower Defense, as long as an equal-price collection of towers is able to handle as many creeps as another collection, those choices should be equally "effective". Consider the following towers:

• $100 - Laser Tower: Deals 100 damage each shot.
• $100 - Slime Tower: Deals 50 damage per shot, but slows the target's speed by 50%.

In isolation, each of these towers can handle exactly the same number and variety of creeps, for exactly the same price. But they go about it in different ways, and their damage is far from identical. When combined with other towers, the valid choices just increase exponentially from there (unless the only choice is a certain combination of towers). Toss in some special creeps, like ones that take 50 less damage each shot or ones that are immune to becoming slowed, and the turrets are meaningfully different just on their own.

Commentsercize

Here's an exercise for you; answer in the comments: What are other ways you could make different types/combos/collections of towers equally effective?

Thesis Work

If you have more time than just a comment and want to practice design, here are some exercises:

• Take a look at some other tower defense games, pick one of them, and analyze the player choices in it, trying to see if they are valid or invalid.
  
• Explain why the valid choices seem to be equally effective.

Onslaught Tower Defence - Part 1

UPGRADES CHOICE

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.

Valid Player Choices

Something important to define is what I mean when I talk about a valid player choice.

All interactive media is comprised of a series of choices made by the end-user: the "player."  Media here comprises everything under the sun that's interactive: from Choose Your Own Adventure books to Dungeons & Dragons sessions with a great GM; from Rock, Paper, Scissors between two children to tournament-level Street Fighter or StarCraft play.

In any piece of interactive media, aside from the emotions the player receives while experiencing the media, the only involvement the player has are the choices he makes.

As a generalization (but in video games especially) the player experience - the range of emotions a player feels from a piece of media - is the basis for the initial draw to a game.  However it is valid player choices that heavily dictates whether an interaction remains enjoyable in the long run (especially simple interactions).

Consider an analogy using a game one may have experienced in elementary school: Rock, Paper, Scissors, Dynamite.  In this variation of Rock, Paper, Scissors, a fourth choice is added - Dynamite - which beats all the other weapons.  While children may find the inclusion of dynamite is exciting and comical at first, the variant quickly loses interest once players realize choosing anything but dynamite is a bad idea.  Their positive player experience from dynamite is eventually offset by the reduced depth of choice in the game.

Having too many choices that are ultimately invalid, or too few seemingly valid choices, can cripple a game's longevity and especially replay value.  No grown adult plays Rock, Paper, Scissors, Dynamite more than a couple times unless as a joke.  Many video game "timeless classics" have myriad and balanced player choices, allowing so much replay a player could possibly never get their fill: Civilization II, Street Fighter, The Sims, etc.  Even classics that rely on strong narratives with seemingly limited choices have many more options buried just under the hood; Final Fantasy VII has only one way to progress through the main storyline, but beyond that the player has numerous options in what abilities they choose during combat and how they customize their equipment, and can even choose different locations to explore later in the game.

So to this end, we shall explore in video games what makes valid player choices, and explore how to avoid making invalid ones.

It Begins

Welcome to Balanced Design.  This blog is dedicated to game design in a general sense, but focuses particularly on balancing the validity of player choices in games.  Secondary interests include getting a job in the video game industry and performing well in that job.

I hope you'll find information here that is both useful and concise.  Feedback is always welcome, and I try my best to answer any questions when time is available.  Feel free to drop me a line!