From the handbook (Page 36):
Built by: Keep
The medium-sized,medium-strength mobile cannon manned by the Cannoneer is an improvement over the Catapult; the damage it deals is much graver. It is however slightly slower and, like the Catapult, isn’t designed for close combat. The Cannonneer can hurl explosive cannonballs over defending walls, unless stationed very close to walls or its target.