A catapult was a siege engine used by the Empire. A sturdy wooden arm with a basket at the end was fastened with a system of ropes and gears to a strong wooden platform, which was mounted on wheels, enabling the catapult to shoot a large ceramic ball placed in the basket a short distance. When a certain rope was undone, the catapult's wooden arm would hurl the ball into the oncoming ranks of the enemy soldiers.
The Empire's catapults caused terrific damage to the Varden's army during the Battle of The Burning Plains. However, Eragon managed to destroy many of them by taking over the minds of the guards standing near the catapults, and forcing them to hack through the ropes which held the catapults' arms in place.
"Catapult" normally describes an entire class of siege engines, including trebuchets, ballistae, onagers, mangonels and others. The engine most commonly identified with the word catapult is, however, the onager. This is a type of torsion catapult, meaning it uses the torsion force of a tightly wound skein or rope to release the projectile. A typical onager consists of a frame to which an axle is mounted. Fixed to the axle is a wooden arm with a sling or a bowl at the end (the bowl was the main feature that distinguished a mangonel from the earlier onager, which featured a sling). In this sling or bowl the projectile is loaded and the arm is forced down against the pull of the rope. When released, the rope's torsion force turns the arm upward in an arc, at the apex of which the projectile is flung from the sling or bowl on a low parabolic trajectory. The entire structure is usually mounted on a wheeled platform for easier transportation.
Onagers and mangonels were relatively easy to build, even on the march, and could be crewed by only a few men, only one of which had to be trained to operate the machine. Depending on the machine's size, it could hurl boulders of several dozen kilograms or clay balls containing "liquid fire", the medieval equivalent of a napalm bomb. The low trajectory and ability to fire incendiary projectiles made these weapons efficient against infantry and cavalry. A mangonel could, thanks to the solid bowl used for holding the projectile, fire grapeshot (i.e. multiple small projectiles, like sharp ingots of lead), causing significant damage in enemy lines. When loaded with stones, multiple engines could be used to breach walls by sustained fire.
Their flat firing trajectory and limited range, along with the impossibility of aiming the machine effectively, made onagers and mangonels unsuitable for bombardment of fortified positions, as the projectile did in the best of circumstances only hit the wall, not the area behind. Also, range and projectile weight were limited, which made attacking a stout wall defended by archers or crossbowmen with these engines a bad idea. Trebuchets were much better suited to this task. Also, the ropes and skeins frequently snapped under the torsion, necessitating the keeping of spares and repair personnel nearby.