The Ballista is the game's first siege and artillery unit, appearing during the Classical Age. This class of units have excellent firepower, but must have support troops to defend them as they have zero or very limited defense on their own. Additionally, artillery units are complex, requiring specialist facilities (the Armoury) and high amounts of time and materials to maintain and construct in a logistical perspective.
Artillery target the enemy's strongest class of units on the field, inflicting damage first and as vehicles. These give the artillery its principle advantages; as the first vehicle damage-class, most infantry units have low vehicle-attacker defence and will crumple under this assault, and gives the advantages of high attack before the enemy can counterattack.
In defensive combat, although these units have low defense it is slightly mitigated by their vehicle-"armour", as this is the first age with vehicles. Most units targeting artillery have low anti-vehicle attack and are forced to destroy each piece one at a time; however, for their cost infantry units are much cheaper, and a loose cavalrymen could destroy several pieces in one round!
The Ballista is best used in defence of your settlements, as they are extremely slow when put on an attacking force (marches that would take 15 minutes with mixed infantry can take 4-6 times as long, often an hour, when these units are brought along). Universal Joints can be researched to improve them, improving their anti-cavalry ability.
They have a faster, light variant called Scorpio, useful in sieges in slightly higher numbers and retaining the Ballista's efficiency; and a slower, heavy variant called Polyboli, which is useless because of its higher upkeep which doesn't match its firepower, and the fact that all Classical troops inflict 2 or more anti-vehicle damage anyway. Barbarians use a variant of Ballistae called Springalds.