Castle Carbonek is the castle in which the Holy Grail is kept.
According to an account told by Shari, in the year 500, King Pelles (the then ruler of Carbonek) and Merlin met with the Irish prince Fergus at Carbonek and persuaded him to lend them the Stone of Destiny, so that they could use it to set up the Sword in the Stone test for the young Arthur Pendragon. ("Rock & Roll")
Carbonek travels about across the face of the Earth, and has no definite point on the map. The Fisher King has limited control on where the castle travels.  Time passes differently at Carbonek, but not in a manner similar to Avalon.  Blanchefleur lives there as its Queen - at least until she joins King Arthur and Sir Griff on their adventures - and Peredur and Duval spend 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.
- Castle Carbonek at Wikipedia, the Free Encyclopedia