Camelot

Camelot was the castle that King Arthur held court. While many places have been theorized to be the location of the legendary court, in truth Camelot was a mobile castle which magically changed location on the whims of the Lord of the Castle.
During Arthur's original reign, it was primarily located in Caerleon though he moved it during his campaigns against the many threats that plagued Britain. Other notable spots included Luguvalium (modern day Carlisle) and Londinium. Following the Battle of Camlann, Camelot was taken into Avalon and this aided the Angles, Saxons, and Jutes to spread across Britain.
Following Arthur's return after the Apocalypse War, Camelot is located squarely in London thanks to its importance as the capital of England. Arthur would retake his role as Lord of the Castle during his reign, though he would hand it off to his replacement when he needed to return to slumber. Following his sacrifice to save Britain and Avalon, his daughter Victoria would take up his role as the Castle's master and Guardian of Britain.