Ballista

A ballista is a siege weapon form of a crossbow. The technical meaning of a ballista is a crossbow powered by torsion as opposed to a tension. What this means is that a ballista is based on the movement of a spring, as opposed to the release of a tight string. Ballistae ranged from small, handheld crossbows to larger ones which could break down walls.

In Mossflower, Timballisto uses a ballista to break down the walls of Kotir, which had already been weakened by flooding. However, in the book, the ballista is described as consisting of counterweights and a throwing arm. To make things more confusing, a current common definition of ballista is a siege engine dart thrower, as opposed to a boulder thrower, which is called a catapult.

Yet in Mossflower, the ballista throws rocks and is refered to as a catapult. Also, it has a release lever (which Dinny apperently enjoyed pulling). It would seem likely, based on the description of the machine, that the ballista was actually a trebuchet. As for the name, there appear to be two possibilities. Either "ballista" is the Redwaller's name for catapults, just like Dibbun is the equivalent of Abbeybabe. Alternitively, it is possible that Brian Jacques made an error in his description.