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.