I've actually found a use for Catapults; I use them to soften up those high-defense troops the AI always sends with it's invasion forces. The UUs Legionary, Impi and Hoplite are particularly hard to take out when your offensive units are Swordsmen and Horsemen, but even ordinary spearmen can be pretty tough when moving on hills or mountains. (As Zouave'd said, this is of course a totally unhistorical use of Catapults.) Presumeably, you can use Cannon the same way against invading Riflemen. Knock 'em down to one HP and let your Cav mop up the survivors. Never really tried, tho' - perhaps the Cannon won't hit offen enough for it to work in practise.