Castle Carbonek - The Castle of the Holy Grail.
Carbonek travels about on the face of the Earth, and has no definite point on the map. Blanchefleur lives there as its Queen - at least until she joins King Arthur and Sir Griff on their adventures - and Percival spends much time there as well.
Real World Background
Carbonek (also known as Corbenic) appears in the medieval Grail romances as the castle of the Holy Grail, and home of the Fisher King. It was extremely difficult to find in the legends, and only a knight pure enough to achieve the Grail could usually gain entrance.