Arthur's World

Arthur's World is an alternate Earth where King Arthur was a historical English king and not merely a shadowy figure of myth. Magic works in Arthur's world, which has produced several exceptional sorcerors, capable of wielding magic even in the main timeline, where magic generally does not work. Arthur's world is accessible from the main timeline, though possibly only if a certain ritual is performed linking the two worlds. The exit point in the main timeline is located in Carbury, in rural England.

The Doctor may have a counterpart in Arthur's world: two separate characters identified the Doctor as Merlin (one, Ancelyn, also knew about regeneration and the dimensionally-transcendent TARDIS interior) though neither had seen him before, and the Seventh Doctor was able to open a door locked and keyed to respond only to Merlin's voice. As the main timeline Doctor has not yet been Merlin (not as of his seventh incarnation, and not seen to be the case since then), he will either become Merlin in the future or was only Merlin in Arthur's world. (DW:Battlefield)