Helepole

a tall (up to 40 meters), multistory movable wooden tower, used in ancient times and in the late Middle Ages for besieging a fortress. The helepole was a complex engineering structure, consisting of a wooden framework divided into floors and having walls of wattle or boards serving as shields. On each floor the walls had loopholes for shooting through. On the higher floors were bridges that could be lowered so that the besiegers could reach the fortress ramparts. The helepoles moved on rollers along planking with the help of levers, tackle, gear wheels, and the like, operated by workmen on the lowest floor, which also housed supplies of materials and a water tank to extinguish fires.