Balancing Four Days and its new enemies
20 Aug 2014

Balancing Four Days and its new enemies

20 Aug 2014

Meet the Jumper, a new invader in the upcoming Four Days: World Defense expansion. Jumpers start out as regular ground invaders…

…but once they’ve been damaged enough, they activate their jetpacks and begin to hover.

While hovering, Jumpers fly too low to be damaged by Skywatch Towers, but they can be damaged by anything that hits both ground and air. Specifically, they can be damaged by Holding Towers, Rocket Towers, and Reaping Towers. And it’s the Reaping Tower that we’re trying to make more useful.

The plight of the Reaping Tower
Reaping Tower
Reaping Tower
Single target, moderate fire rate, hits air and ground, damage scales with enemies it defeats

In the 2 years since Four Days was released, we’ve talked to a lot of people, seen a lot of high scores, and played a lot of it ourselves. And while we feel that all 9 towers are generally balanced very well, the Reaping Tower is still used less than we’d like.

It was designed to be a hybrid tower, effective at defeating invaders on the ground and in the air. We thought about it in terms of World of Warcraft, where druids have historically been good at lots of things but not the best at anything. We couldn’t make Reaping Towers better at defeating air invaders than a Skywatch Tower, and we couldn’t make them better at defeating ground invaders than a Gun Tower.

So, much like druids, we gave them a different kind of bonus: efficiency. With some planning and some luck, Reaping Towers can be a better ground investment than Gun Towers — while still helping to defeat air invaders, too!

Skywatch Tower
Skywatch Tower
Single target, rapid fire rate, hits air, each shot does moderate to high damage

It’s the “some planning and some luck” part that ends up being a problem. Reaping Towers become stronger whenever they get the killing hit on an invader. This is easy to plan for early in the game, when you have very few towers and you can upgrade your Reaping Tower before everything else. But later in the game, when there are lots of towers on the grid, it’s much more difficult to ensure that your Reaping Tower will get enough killing hits to power up and become an efficient investment.

And so the Reaping Tower ends up being a good choice in the early game, and a very unpopular choice in the late game. No other tower suffers from this issue. So how do we fix it without upsetting the overall balance of the game?

Balancing a tower defense game

This post is actually the first in what will be a series about overall game balance in Four Days. Today’s topic, if you haven’t already figured it out, is tower balance. What makes a tower good? More importantly, what makes a tower TOO good? And if major game studios with huge budgets still have to release balance patches all the time, how can indie developers expect to get it right? As we work on the upcoming expansion for Four Days, I’d like to highlight some of what we’ve learned in making our own tower defense game.

Tower balance: cost efficiency
Gun Tower
Gun Tower
Single target, rapid fire rate, hits ground, each shot does low damage

When I say that Reaping Towers can be “more efficient” than Gun Towers, I’m referring to cost efficiency: the amount of damage per second (DPS) a tower will do, versus the cost of building and upgrading the tower. The point of a tower defense game is typically to do as much damage to invaders as possible while working with a limited resource budget, so cost efficiency is a major consideration.

We started with the Gun Tower: it costs $100 to build, and it’s the DPS standard that other towers should measure up against. If a tower is less cost effective than a Gun Tower, it should have some other benefit. If it’s more cost effective than a Gun Tower, there should be a drawback.

The “single type” test and damage efficiency

One particularly useful balance test is to see how things play out if you only use one type of tower. For us, playing the game with nothing but Gun Towers was an illuminating experience. A map filled with Gun Towers was much, much better than a map with a combination of Gun Towers and other damage towers.

We built damage metrics and logging into the game during development so we could see which tower types were doing the most damage, but we didn’t account for the OTHER kind of tower efficiency: damage efficiency. Imagine two towers: a Gun Tower that fires once per second and does 10 damage each time, and a Rocket Tower that fires once every 5 seconds and does 50 damage each time. Mathematically, both of these towers do 10 damage per second.

Rocket Tower
Rocket Tower
Single target, very slow fire rate, hits ground and air, each shot does massive damage

Now imagine a line of invaders marching through a game level, each with 20 hit points. The Rocket Tower will kill one of these invaders every 5 seconds, doing 50HP of damage to an invader that only had 20HP to begin with. The extra 30HP of damage does nothing, and is essentially wasted damage. But in that same 5 seconds, the Gun Tower would kill 2 entire invaders and injure a third. Gun Towers distribute their damage very efficiently, and it means their effective DPS is much higher than a Rocket Tower’s effective DPS, even though both have the same theoretical DPS.

We toned down Gun Towers as a result, but we also made Rocket Towers better. Now Rocket Towers are particularly effective against invaders with lots of HP, but less effective (relative to a Gun Tower) against weaker invaders with little HP.

Positioning and stacking
Flame Tower
Flame Tower
Multiple target, continuous fire rate, hits ground, applies burning effect that does damage over time

Here’s a secret: Four Days was written by two people, and we both acknowledge that Flame Towers are the best tower. Flame Towers do a moderate amount of direct damage to every invader they hit, along with fairly heavy damage over time (DoT) afterward. Their cost efficiency and damage efficiency are both very high.

So how do they fare in the “single type” test? Poorly! And it’s because the DoT effect doesn’t “stack.” If five Flame Towers are all hitting the same group of invaders, those invaders will take direct damage from all five Flame Towers, but the DoT effect is only applied once. Putting one Flame Tower down is great, but putting two Flame Towers down next to each other is wasteful.

And that highlights one of the most important aspects of Four Days: positioning. Designing a tower defense game is all about giving the player interesting choices. They choose which towers to use, which towers to upgrade, and where to place them. Flame Towers are most effective when placed just far enough apart to make sure invaders are always being burned by the Flame Tower DoT. So Flame Towers obviously benefit from positioning… how about the other towers?

We’ll get to that in the next blog post. Check back next week for the rest of this discussion!

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