- A siege engine is a device that is designed to break or circumvent city walls and other fortifications in siege warfare. Some have been operated close to the fortifications, while others have been used to attack from a distance. From antiquity, siege engines were constructed largely of wood and tended to use mechanical advantage to fling stones and similar missiles. -- Wikipedia on Siege Engines
In Kingdoms of Camelot, siege engines are a type of troops which includes Ballista, Battering Rams, and Catapults.
Siege engines are particularly susceptible to attack from the defensive units Spiked Barrier and Defensive Trebuchet.
WARNING: if you win the item "Siege Volunteers" from Merlin's Magical Boxes , be wary that the new troops will eat around 90,000 food per hour.