The Hatra Ballista was a Roman siege catapult powered by torsion springs consisting of rope bundles made from animal sinew. This machine was particularly innovative, in that the two wooden throwing arms rotated inward, rather than outward as on conventional catapults. The inward swing allowed for a greater angle of rotation, which increased the elastic energy stored in the springs. We know about the Hatra Ballista because of archeological remains found in Hatra, Iraq. Yes, the Romans fought there too!