Roman siege engines BC, and surrounded by Roman fortifications. As was standard siege technique at the time, "ballistae" were placed up in the towers with other soldiers armed with either bows or slings. The onager was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an onager (wild ass). It is a type of catapult that uses torsional pressure, generally from twisted rope, to store energy for the shot. The onager consisted of a frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed;
Onager (weapon) These he says should be transported fully assembled on ox carts to ensure readiness in case of sudden attack, in which case the onagers could be used for defence immediately. For Vegetius, the onagers were stone throwing machines. In the late 6th century the Avars brought the Chinese traction trebuchet, otherwise known as the mangonel, to the Mediterranean, where it soon replaced the slower and more complex torsion powered engines. Onager (weapon) The onager (British /ˈɒnədʒə/, /ˈɒnəɡə/, U.S. /ˈɑnədʒər/) was an imperial-era Roman torsion powered siege engine; in other words, a small catapult. The onager was first mentioned in 353