Onager

• An onager operates on simple spring technology. A long flexible wooden or metal arm is firmly fastened on one end to a carriage or other mobile platform. At the other end, a large bowl-shaped receptacle holds the ammunition. A crank with lengths of rope attached pulls the arm down, bending it until it resembles a large arc. When the rope is released, the energy stored in the bent arm launches the rock, flaming object or other payload at the defenders.
  
  

Trebuchet

• The design of a trebuchet is more complex than a simple catapult. A trebuchet relies on a counterweight to drive the arm of the siege engine. Instead of a large receptacle, a trebuchet employs a sling to carry and hurl the payload. The counterweight swings down through the frame of the trebuchet as the sling, attached to the long arm, gathers speed. At the top of the arc, the sling hurls the payload at the defenders.
  
  

Ballista

• A ballista resembles a giant crossbow. Ballistae launched large arrows or darts, rather than rocks and boulders, and were used both on ships and on land. Warships with mounted ballistae fired flaming ironclad arrows or bolts at the hulls and sails of opposing ships.
  
  

Drawbacks of the Siege Engine

• Lack of mobility made catapults suitable for attacks against fortified locations or emplacements but not against moving armies. These were large and heavy devices that required many people and animals to move them into place. In some cases, the devices were built on the spot. The advent of gunpowder and cannons, with their smaller footprints and higher mobility, made catapults obsolete as siege weapons.