Two entries with the name Giant’s Tower.
Giant’s Tower
Tour aux Geants
A tower near the Spring of Healing, ruled by Atamas, who was also known as the Knight of the Tower.
Atamas used the healing powers of the Spring to defeat all knights who came his way, imprisoning them in the Tower. Among its prisoners were Gawaine, Gaheris, Bleoberis, and Sagremor (Sagramore). The Giant’s Tower was liberated by Palamedes (Palomides), who defeated Atamas during the Grail Quest.
In Joseph of Arimathea’s days, the Tower had been ruled by King Camalis, who imprisoned Nascien and Mordrains.
Source
Post-Vulgate Queste del Saint Graal | 1230-1240
Giant’s Tower
The entrance to Arthur’s palace in Caerleon. Arthur rallied his warriors there at the beginning of the Roman War.
Sources
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Thomas Castleford’s Chronicle | Thomas Bek of Castleford, 1327
Alliterative Morte Arthure | c. 1400