Catapults are by far the most common siege weapon of the Royal armies (as well as other medieval forces). The Rainbow Knights took this fascination to the extreme, even mounting catapults on horses, prison wagons and the like.
The most common ammunition for catapults are rocks. Various other payloads have been tried, such as fireballs and flamewheels, depending on the spirit of the times.
The only weapon that comes close to rivalling the catapult's popularity is the ballista, which recently was deployed impulsively in massive numbers by the Troll armies.
Catapults are the simplest form of artillery, a more broad and encompassing category.