A siege tower is a specialized siege engine, constructed to protect assailants and ladders whilst approaching the defensive walls of a fortification. The tower was often rectangular with four wheels and a height roughly equal to that of the wall or sometimes higher to allow archers to stand on top of the tower and fire into the fortification.
Used throughout antiquity in both the Far East and Europe, siege towers were of unwieldy dimensions and therefore mostly constructed on site of the siege. Taking a lot of time to construct, siege towers were mainly constructed if the defense of the opposing fortification could not be overcome by ladder assault, by sapping or by breaking walls or gates.
The siege tower sometimes housed knights, or crossbowmen who shot arrows at the defenders. Because of the size of the tower it would often be the first target of cannons or any other heavy projectiles.
Siege weapons were varied and ingenious inventions, their main object being to effect an entrance through the gates or walls.
At the siege of Marseille the defenders countered attempts to tunnel under their walls by digging a large basin inside the walls which they filled with water.
Onagri were used in sieges to batter down walls, as well as by defenders to smash siegetowers and siege works.