The onager was a Roman torsion powered siege engine. It is commonly depicted as a catapult with a bowl, bucket, or sling at the end of its throwing arm. The onager was first mentioned in 353 AD by Ammianus Marcellinus, who described onagers as the same as a scorpion. The onager is often confused with the later mangonel, a "traction trebuchet" that replaced torsion powered siege engines in the 6th century AD.
Onager with a bowl bucket
Sketch of an onager with a sling, a later improvement that increased the length of the throwing arm, from Antique technology by Diels.
A siege engine is a device that is designed to break or circumvent heavy castle doors, thick city walls and other fortifications in siege warfare. Some are immobile, constructed in place to attack enemy fortifications from a distance, while others have wheels to enable advancing up to the enemy fortification. There are many distinct types, such as siege towers that allow foot soldiers to scale walls and attack the defenders, battering rams that damage walls or gates, and large ranged weapons that attack from a distance by launching projectiles. Some complex siege engines were combinations of these types.
Replica battering ram at Château des Baux, France
Siege engine in Assyrian relief of attack on an enemy town during the reign of Tiglath-Pileser III 743-720 BC from his palace at Kalhu (Nimrud)
A stone-throwing machine set to defend a gate, in the fresco of Guidoriccio da Fogliano by Simone Martini (14th century).
The medieval Mons Meg with its 20" (50 cm) cannonballs