From Manners and Customs of the Bible:

The invention of these engines of war marks an era in warfare, since by their use the power of an army was greatly increased whether for attack or defense. They were simply machine bows and slings, which, by the application of mechanical principles, were made to throw heavier projectiles than the smaller weapons which were held in the hand. We have here doubtless the origin of the balistae and catapult which afterward became so famous in Roman warfare. The balista was used to shoot stones; the catapults projected darts. Historians mention three sizes of balista, which were graded according to the weight of the stones they threw, namely: a half hundred weight, a whole hundred weight, and three hundred weight. Occasionally there were some used which threw stones as light as two pounds. Several balls of limestone, which were found in the excavations in Jerusalem in 1869, are thought to have been used as missiles and hurled from a balista. Catapultae were denominated according to the length of the darts thrown from them. No exact idea can now be had of the forms of these engines. The Romans classified them under the generic title of tormentum, because of the twisting of the hairs, thongs, and vegetable fibers from which the elastic string was made which gave impetus to the projectile. See Smith's Dict. Class. Antiq., s. v. Tormentum. These engines were often used from the top of a “mount” or inclined plane. See note on Ezekiel 4:22And lay siege against it, and build a fort against it, and cast a mount against it; set the camp also against it, and set battering rams against it round about. (Ezekiel 4:2) (#565).