Royal fortress on the northern bank of the River Thames.
The central keep, or donjon, known as the White Tower because of its limestone, was begun 0441; 1078 by William I the Conqueror inside the Roman city wall. In the 12th13th century the fortifications were extended beyond the wall, the White Tower becoming the nucleus of a series of concentric defenses. The only entrance from the land is at the southwestern corner; when the river was still a major highway, the 13th-century water gate was much used. Its nickname, Traitors' Gate, derives from the prisoners brought through it to the Tower, long used as a state prison; many were murdered or executed there.