Ballista

A ballista is one type of siege engine which uses a crossbow-like arrangement to hurl a projectile, as opposed to a catapult which throws its projectile by employing a long arm structure.

The crossbow-like arrangement itself consists of a pair of opposed rigid arms, each with torque provided by their own torsion bundle or skein of twisted rope.

External Links:

Ballista Construction

"Baby Ballista" and "Ballista Jr." at Seige-Engine.com

See also: Arbalest