While the core ballista mechanism was (probably) developed by the ancient Greeks by 5th century BC (in forms like oxybeles and gastraphetes), there is no doubt that the Romans advanced the practical scope of such fascinating weapon systems, along with their deployment and usage on ancient battlefields. To that end, the manuballista was a Roman siege engine (originally based on the design of Heron of Alexandria) that has often been described as the “most advanced two-armed torsion engine used by the Roman army” (circa late 1st century AD).
The so-named carroballista was an extension of the similar manuballista technology, but its difference lied in its advantage of maneuverability. In essence, the weapon system was developed as a cart-mounted ballista, thus entailing a type of mobile field artillery. ArcheoArt has described the weapon in some details, based on the reconstruction of Michael Lewis –
The caroballista: a powerful descendent of the Roman ballistae and catapultae. This two-man example is being used at some point in the Dacian War. It shoots heavy bolts, and is an extremely powerful weapon, thanks to the wide sweep of the arms, which transmit a huge amount of stored spring-energy to the ammunition. The sinew-loaded spring frames are made of iron, and have tough leather covers to protect them from enemy fire- and the weather. The machine is mounted on a universal joint, atop a stand, and can be pointed in any direction. To shoot, one man turns the windlass to draw back the slider and rope, while his crew-mate holds it steady, and places a bolt on the slider; he then holds the tiller and aims, while the first pulls the trigger-bar. The whole weapon is light enough for its two-man crew to move it around and load it onto a cart when the division has to move; in this way, it is the equivalent of a WW2 Bren-gun.
According to Vegetius, the late Roman army adopted the carroballista as a standard mobile field artillery, with 55 carroballistae being the standard number that was distributed among each legion. Each of these war machines, having the advantage of light yet stout metal spring-frame, were transported by mules and operated by contubernium units (group of ten legionaries).