The Hive Cluster Is Under Attack!

The Hive Cluster Is Under Attack:
Unit Type Balance in StarCraft and StarCraft: Brood War

Part of the problem here is that I don't have a very good name for the concept I want to talk about. I'm sure there's an official military term, but, as is often the case, I don't know what it is, and can't be bothered to find out.

Anyway, what I'd like to do is start with an example that shows the concept in action. Suppose I assemble an attack force of consisting solely of siege tanks. Because siege tanks can only attack ground-based units, the force will be vulnerable to attack by air; not only vulnerable, but completely unable to retaliate. If, realizing this, I add some goliaths (ground-based units having a strong air attack) to the force, the force will be improved, but it will still be vulnerable—in this case, to attack by cloaked (invisible) units. If, however, I now add a single science vessel, which can detect cloaked units, the force is complete. It can be overwhelmed by a numerically superior force, but it's not vulnerable to any particular unit type.

Now let me try to state the concept in general. Just as the elements in Rock, Scissors, Paper have interactions with each other, so do the unit types in StarCraft, except that the interaction of unit types need not be all-or-nothing. As a result, there are certain unit types that work well together, that aren't overly susceptible to any particular form of attack. The problem of determining these unit types I think of as the problem of unit type balance, or of composition of forces.

It would be fun to try and quantify the problem. I think the way to start would be to compute, for each pair of unit types, the amount of damage that can be inflicted per unit time, as a fraction of the target's hit points. That would make a nice matrix to which one could apply game theory or linear programming or whatever. However, when you think about including area-effect weapons, differential recovery rates, and special abilities, not to mention production costs, it all gets very complicated.

The concept of unit type balance doesn't apply only to StarCraft, of course. It started becoming clear to me when I was playing Myth, and I may even have gotten a hint of it from Strategic Conquest. I'm sure it applies to lots of other, similar computer games, and I imagine it applies to real warfare as well.

In terms of the continuum between strategy and tactics, I think of the concept as being more on the strategic side. There are tactical aspects, such as which unit types to bring to the fore at which times, or how to cripple a force by selective destruction of unit types, but I see those as separate topics.

Now let me throw in another example. In the original game, the marine wasn't all that useful as a unit type—marines were easy to produce, but had few hit points relative to all the mechanized units, and could never recover after being wounded. In Brood War, however, a few more unit types were added, and one of these was the medic. Medics can heal marines, and quickly, too. As a result, a force of marines plus medics is now a viable primary attack force, one that's easier to produce than a force of siege tanks and goliaths. (If you're familiar with the game, you might be amused to know that a force of marines and medics is highly susceptible to defilers—the natural behavior of the medics is to waste all their healing power while the venom is still active.)

Thus, to state the obvious, the balance of unit types depends on which unit types are available. What's a little less obvious is that the balance depends on lots of other things as well. No, that's not quite right … it seems to me that the balance depends only on the unit types, but that the composition of a particular force depends on lots of other things. It depends, for example, on whether the force is to be used for attack or defense; for an attacking force, it depends on whether the force is to be ground-based, air-based, or perhaps mixed, for an island invasion; and so on. It depends, in short, on the objective.

I suspect that part of what makes StarCraft appealing is that there's enough variation in the available unit types and objectives to create lots of different composition problems. In the original game, the variation in unit types was due mainly to the types being introduced a few at a time, and could have been accidental, but in the sequel it was clearly intentional—there are a couple of scenarios in which the types are more or less arbitrarily constrained, by removing, say, all the air units. (Speaking of constraints, I'm reminded of Hofstadter's argument in Sparking and Sparkling, Thanks to Constraints that constraints are one of the things that drive creativity.)

Finally, I have to point out that I wasn't telling the whole truth when I said that the balance depends only on which unit types are available. You have to take “available” to have two meanings, “available to you” and “available to your opponent, or opponents”. Against an opponent with no cloaked units, for example, no science vessel is necessary.