Download Catapult Kit FREE Unity. 

Very Detailed Medieval Catapults. Great for a game genre like Tower Defense, MMORPG, strategy etc. Wheels on cannon can be rotated individual, so you can animate it easy. and the same with the thrower arm, pivots are all set.

There are four types included, catapult with wheels, and for solid ground, and both types with extra armor for arrow defense etc.