The Ballista was used in Roman times and it was a huge piece of armory in the form of a crossbow which fired a heavy bolt of distances ranging up to 500 metres.
It had two arms at the side which were wound back to load the weapon then fixed with heavy ropes or twisted animal sinews.
When the bolt was placed in the correct position the weapon was fired by releasing the retaining catch.
This allowed the block to slide quickly forwards as the ropes unwound, making the bolt/heavy stone or rock fly out the front of it through the air at a quick speed causing a large amount of damage wherever it landed.