Three entries with the name Red Castle.
Red Castle
Vermeil Castle
A castle conquered from Lord Serses by Sir Bors, who established a monastery on the site.
Source
Vulgate Lancelot | 1215-1230
Red Castle
When a lord named Elain died, he left the Red Castle to his maiden daughter. Yvain the Black wanted to marry the maiden, and he besieged the castle when she refused. Guiron the Courteous came to the maiden’s aid, defeated Yvain, and lifted the siege.
Source
Palamedes | c. 1240
Red Castle
The home of the knights Hugh (Hue of the Red Castle) and Edward of the Red Castle. The knights had extorted a barony from the Lady of the Rock.
This probably was somewhere in the marches of Wales.
Source
Le Morte Darthur | Sir Thomas Malory, 1469-1470