The ballista was an ancient missile weapon which launched a large projectile at a distant target. Developed from earlier Greek weapons, it relied upon different mechanics, using two levers with torsion springs instead of a prod, the springs consisting of several loops of twisted skeins. Early versions ejected heavy darts or spherical stone projectiles of various sizes for siege warfare.
The Greek ballista was a siege weapon. All components that were not made of wood were transported in the baggage train. It would be assembled with local wood, if necessary. Some were positioned inside large, armored, mobile siege towers or even on the edge of a battlefield. For all the tactical advantages offered, it was only under Philip II of Macedon and even more so under his son Alexander, that the ballista began to develop and gain recognition as both siege engine and field artillery.
After the absorption of the Ancient Greek city-states into the Roman Republic in 146 BC, the highly advanced Greek technology began to spread across many areas of Roman influence. This included the hugely advantageous military advances the Greeks had made (most notably by Dionysus of Syracuse), as well as all the scientific, mathematical, political and artistic developments.
The Romans 'inherited' the torsion powered Ballista, which had by now spread to several cities around the Mediterranean, all of which became Roman spoils of war in time, including one from Pergamum, which was depicted among a pile of 'trophy' weapons in relief on a balustrade.
The torsion ballista, developed by Alexander, was a far more complicated weapon than its predecessor and the Romans developed it even further, especially into much smaller versions, that could easily be carried.
Polybius reports about the usage of smaller more portable ballistae, called scorpions, during the Second Punic War.
With the decline of the Roman Empire, resources to build and maintain these complex machines became very scarce, so the ballista was supplanted by the simpler and cheaper onager and the more efficient springald.
Though the weapon continued to be used in the Middle Ages, it faded from popular use with the advent of the trebuchet and mangonel in siege warfare. The crossbow and the longbow supplanted it as sniper weapon. They all were simpler to make, easier to maintain, and much cheaper.