In Galactic Civilizations II, you have beam weapons, missile weapons, and mass-driver weapons.

Against these weapons are corresponding defenses -- shields, anti-missile defenses, and armor.  Defenses in a different area do provide some benefit -- they are the square root of their value.

So if a fighter with Blasters (beam) of 3 attacks a ship with 0 shields, 4 armor, and 0 point defense then the battle goes something like this:

Fighter rolls beteween a 0 and a 3 and the defender rolls between a 0 and a 2 (square root of 4).

The significance of this is that if you pick the wrong defense, you take quite a pounding on your ship.  This means that defenses should cost quite a bit less and be somewhat easier to obtain (otherwise you might as well just crank out the strongest offensive ships possible).

Balancing this out is tough.  We want different defenses to have different advantages over time.  Here's what we came up with: