Castle Britannia was the home of Lord British and served as his seat of power. It is located in the capital city of the Britannia, Britain, and is one of the largest structures in Ultima Online. It features a throne room, kitchen, stables, barracks, gardens, smithy, library and even a hidden laboratory. This is all surrounded by a moat and the exterior castle walls are thick enough to traverse through.
After Lord British departed the realm, Castle Britannia continued to serve as the seat of power for the Ruling Council. As such, many in-game events either originate from or can involve a visit to this vast structure.
Castle Britannia is featured prominently in several of the prior, single-player Ultima series games although the appearance, size and architectural layout has changed significanctly in each iteration.
Cannons were originally only seen on the ramparts of Castle Britannia before making an appearance when the town of Haven was created. Subsequently, cannons have also appeared in some in-game events, such as the Magincia Invasion, and Decorative Cannons are now obtainable by players as 7th year Veteran Reward.
Using a sextant will reveal that the coordinates 0°0'S,0°0'E are located on the very spot of the throne in the throne room of Castle Britannia. Until sometime in 2003, players could approach and sit in the throne itself. It was blocked off when Event Moderators began using the location extensively for their live events.
Certain notable NPC's, such as Sherry the Mouse, purportedly make their home in Castle Britannia.
While often called simply "Lord British's Castle" or "Castle British", the proper name is in fact Castle Britannia.