A catapult is a large siege weapon used to bring down castle walls and other fortifications, as well as attack ground units. They are slow, and have a long reload time in between volleys, much like that of the Ballistae. Catapults can do a very large amount of damage to ground units, as well as attack Scorpions, which are impervious to arrows. Catapults generally fire large stones that do a massive amount of damage to stationary infantry, as well as fire pots which can do a number of different things. Catapults are not good melee units, and should not be used as such. It would be wise to use Catapults as an exclusively supporting unit.
Catapults launch large stones and fire pots. They are slow, but can inflict tremendous damage to all enemies on the ground and collapse castle walls.