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Unlike other alternative timelines, Arthur's world was a natural alternative that had sprung from the universe. (PROSE: No Future) On this Earth, King Arthur was a historical British king and not merely a shadowy figure of myth. It produced several exceptional sorcerers (or users of advanced psychic powers).
Arthur ruled here before his overthrow by Morgaine and her son, Mordred. Morgaine imprisoned Merlin, a version of the Doctor himself, leaving the knight Ancelyn, loyal to Arthur, and others to survive. Ancelyn knew of TARDISes and recognised the Doctor's seventh incarnation, though not by his face. The Doctor theorised that one of his future incarnations would move to Arthur's world. This incarnation left a note for him.
Arthur's World was accessible from the Doctor's universe. The exit point in the main timeline was located in Carbury near Lake Vortigern, in rural England. An interstitial vortex linked the dimensions.