Used in ancient times, the siege tower was a four-sided, wheeled wooden tower designed as a siege engine to protect assailants when they were approaching the defensive walls of a fortress. Since the towers were wooden and thus flammable, they had to have some non-flammable covering of iron or fresh animal skins. Although they were mainly made from wood, sometimes they had metal parts. The tower had a square base and tapered towards the top. The siege tower moved on four wheels and had 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.
Although the first known siege towers were used by the armies of the Asyrian Empire, they were extensively used by the Romans. With the collapse of the Roman Empire in the West into independent states, and the Eastern Roman Empire on the defensive, the use of siege towers reached its height during the medieval period. Siege towers also became more elaborate during the medieval period; at the Siege of Kenilworth Castle in 1266, for example, 200 archers and 11 catapults operated from a single tower. Even then, the siege lasted almost a year, making it the longest siege in English history.
Siege towers became vulnerable and obsolete with the development of large cannon. They had only ever existed to get assaulting troops over high walls and large cannon also made high walls obsolete as fortification took a new direction.