Below is a model of an onager catapult at the Museo della Civilta in Rome. This machine would be the heavy artillery to the ancient world. The handles to the left (rear of the catapult) are in fact levers by which the soldiers would wind the throwing arm back. On the right you see a cushion at the front of the catapult. No doubt it was there to soften the blow of the throwing arm and so help to prevent the machine from tearing itself apart.