Helepolis is the Greek name for a movable siege tower.
Model of a Helepolis siege tower, Thessaloniki Science Center and Technology Museum
Naval helepolis. Two quinqueremes having a helepolis on their decks.
A Roman siege tower or breaching tower 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 with its height roughly equal to that of the wall or sometimes higher to allow archers or crossbowmen to stand on top of the tower and shoot arrows or quarrels 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.
12th century siege of Lisbon with siege tower, trebuchets and mantlets.
Assyrian attack on a town with archers and a wheeled battering ram; Neo-Assyrian relief, North-West Palace of Nimrud (room B, panel 18); 865–860 BC
The remains of the Roman siege-ramp at Masada
Medieval English siege tower