The ballista fires fiery arrows at enemies and buildings from very long range. Arrows from the ballista cause great impact damage to most units and siege engines. They are also capable of setting buildings on fire on low health (but incapable of setting pitch on fire).
Tower ballistae can only be built on larger stone towers - bastions and larger - that do not have hoarding on top. This machine is operated by two engineers, who wind up the string, insert the bolt, then fire. The crew automatically comes upon placement.
Ballistae see an extended usage against lined-up and densely packed units, as their arrows knock back units in the same way as catapulted rocks do. They can knock units into a river or trees, killing units instantly and they are also useful at removing a knight's mount.
Ballistae are quite unreliable to an extent, as they can sometimes lose track on units that get out of range. Retargetting them manually can solve the problem.