A
siege engine is a device used to attack a fortified target during a
siege. From ancient times up until roughly the invention of the
cannon, fixed
fortifications such as large walls, trenches and moats were quite effective at holding off a purely
infantry-based assault.
City walls were even able to do a credible job of withstanding the attack of foot soldiery if they were properly manned. The attackers, therefore, soon turned to technology for the answer.
It's difficult to pin down when the first siege engines were brought to bear. Ancient China seems to have had an early start with traction trebuchets and the like. The Western world's image of siege engines is typically that of a medieval nature - European castles and towns withstanding the onslaught of neighbors or barbarians (never mind that the Great wall of China did it all first and in grander style).
So what are siege engines?
There are two basic types - those intended to directly damage or attack the fortifications, and those intended to assist in engineering efforts. Among the latter are the siege tower and the basic shelter- used to protect those working at digging through walls or preparing ramps against attack from the defenders.
The more destructive (and hence more famous) types are many. The basic task of throwing a heavy and destructive object can be approached in many ingenious ways. Among them are the onager (a sling-based throwing engine powered by torsion), the mangonel (essentially an onager with a cup instead of a sling to allow for different payloads), the ballista or siege crosssbow, the catapult which any watcher of Wile E. Coyote is familiar with, and the various types of trebuchet. All of these, however, are predated by the basic battering ram.