A ballista deployed on the field
Ballistae were originally field artillery that were used against small groups of infantry and mechanical devices. The ballista works similar to a catapult, but instead of a pressable holder, it features a string mechanism resembling a giant crossbow. The string can be pulled backwards to fasten in a very taut way to store enough kinetic energy that when released, its projectile can fly with great speed.
Ballistae most often used javelins and stakes scaled to the device's size as ammunition. While heavy to move around and its deployment could take significant time, it made up for the fact that one well-placed shot could cause significant damage to enemy ranks. Ballistae were best used in dispatching advancing formations and damaging siege engines.
In medieval times, the ballista remained an excellent siege weapon, as well as a defense utility for its original purpose: dismantling personnel and mechanical equipment alike.