Siege towers were large wooden structures. They were wheeled into position next to the castle so that attacking soldiers could climb up the rear of the tower and attempt to cross over to the castle walls. The front and sides of a siege tower were usually covered with animal hides so that defending soldiers could not easily set it on fire. Catapults throw projectiles, such as large rocks, over or against castle walls. They could throw large stones well over 300 meters, which was beyond the reach of defenders' arrows so that attacking soldiers could use catapults in relative safety.

Although the first known siege towers were used by the armies of the Syrian 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 cannons. They had only ever existed to get assaulting troops over high walls and large cannons also made high walls obsolete as fortification took a new direction.