Roman siege technology was inherited largely from the Greeks and Carthaginians, but Rome did with siege engines what Rome did with everything: standardized them, integrated them into the regular army, and produced them at scale. By the early Empire, every legion travelled with its own artillery train, and a Roman commander could expect to put dozens of torsion-powered machines into action at any major siege.
Ballista
The ballista was a two-armed torsion-powered missile thrower — the Roman equivalent of heavy artillery. Two horizontal arms, each anchored in a tightly twisted bundle of sinew or hair, drew a bowstring back against enormous stored energy. When released, the arms snapped forward and the string flung a stone or a heavy bolt at distances that could exceed 300 to 400 yards.
Ballistae came in many sizes, from machines small enough to be wheeled into position by a few men to enormous siege pieces that required dedicated platforms. Smaller versions threw bolts the size of spears; larger ones threw stones weighing up to a talent (roughly 57 pounds) capable of breaking down walls or smashing through siege towers. Vegetius, writing in the 4th century AD, claimed that each Roman century — roughly eighty men — was equipped with its own ballista.
The Greek origin of the design shows in the technical vocabulary — the Romans kept using Greek terms for the parts — but Roman engineers refined the construction substantially. The most important refinement was metal frames: replacing the wooden frames of earlier ballistae with iron ones increased the torsion the machines could store, which increased range and power. By the late Empire, the ballista had evolved into the smaller, more portable carroballista, mounted on a cart and operated by the legionary artillery.
Onager
The onager was a single-armed torsion catapult used primarily for hurling stones. A single heavy wooden arm was anchored at its base in a horizontal bundle of twisted rope or sinew. The arm was winched down to the horizontal, a stone (or, in less refined moments, a corpse, a beehive, or a flaming pitch ball) was placed in a sling at its tip, and the trigger released the arm to swing violently upward against a padded crossbar. The stone, released from the sling at the top of the arc, flew in a high parabola toward the target.
It got its name from the Latin for "wild ass" — onager — because of the violent kick it gave when fired. The machine recoiled hard enough that operators had to brace it with rocks or anchor it to the ground; otherwise it would jump several feet on every shot. The rate of fire was slow, perhaps one or two stones a minute even with a trained crew.
Onagers were primarily late Roman weapons — they appear in the historical record from roughly the 3rd century AD onward and were used heavily by the Eastern Roman (Byzantine) army into the medieval period. They were less precise than ballistae but threw heavier loads, and their high arcing shots could clear walls that flat-trajectory ballistae could not. Roman armies on the move sometimes carried disassembled onagers in pieces, reassembling them on site — the timber alone was too heavy to transport intact across a province.
Scorpio
The scorpio was the Roman army's anti-personnel weapon: a smaller two-armed torsion machine that fired heavy bolts with extreme accuracy at distances of 100 to 200 yards. Functionally it was a heavy crossbow on a stand, but built with the same torsion-powered mechanism as a ballista — meaning it threw a roughly two-foot iron-tipped bolt with enough force to drive through shield, armor, and the man behind both.
What set the scorpio apart was its precision. Unlike the area-effect onager or the heavy siege ballista, the scorpio was aimed at individual targets. Caesar's commentaries on the Gallic Wars repeatedly mention scorpiones picking off enemy soldiers from the walls of Roman camps and forts; on more than one occasion he describes besieging Gauls being shot through the head from across a fortification.
By the early Empire, the scorpio was standard-issue. Vegetius, again, claimed each century carried one — meaning a single legion of roughly five thousand men deployed about sixty scorpiones in addition to its larger artillery. The cumulative effect on a battlefield must have been substantial: dozens of accurate, hard-hitting bolt-throwers picking off enemy officers and standard-bearers from beyond the range of any answering bow. The scorpio is a reminder that the Roman edge in siege warfare was not just bigger machines — it was the integration of artillery into the structure of the regular army.