A siege tower (or in the Middle Ages a belfry) is a specialized siege engine, constructed to protect assailants and ladders while 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. Because the towers were wooden and thus flammable, they had to have some non-flammable covering of iron or fresh animal skins. The siege tower was mainly made from wood but sometimes they had metal parts. The siege tower sometimes housed pikemen, swordsmen, or crossbowmen who shot quarrels at the defenders. Because of the size of the tower it would often be the first target of large stone catapults but it had its own projectiles with which to retaliate. Siege towers were used to get troops over an enemy wall. 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. (from wikipedia.org)