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. The target of the siege would usually not see the siege tower approaching and would be caught off guard. 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, but it had its own projectiles to fight back with.

Siege Towers were sometimes used to enable millitary troops to enter enemy fortifications. When a siege tower was near a wall, it would drop a gangplank between it and the wall. Troops could then rush onto the walls and into the castle or city.