Siege-towers were towering, powerful and effective siege machines used by Mordor in TA 3019. They were designed to protect assailants and ladders while approaching the defensive walls of a fortification. They were manned by as many as four trolls because of their great size, and garrisoned many dozens of Orcs within, prepared for onslaught.
An army of Mordor used siege-towers in Gondor, the evening before the Battle of the Pelennor Fields, to breach the high walls of Minas Tirith and invade the vast city. Many were destroyed within the night by the walls' defenders. It was Grond instead that soon allowed a major entry for Sauron's army into Minas Tirith.