There is no basis for historical accuracy when you are talking about a legend. Arthur actually existed in the middle ages, but the story had been retold and changed so many times that there is no longer any basis for historical accuracy. The same is true for other legendary characters: Daniel Boone, Davey Crockett, Jesus, Noah. All of these people existed but they did not perform all of the miraculous feats that have been attributed to them. But it is fun to tell these tales and you can put your own spin on them, just as the creators of the "Merlin" series have done. In my opinion the gold standard of the Arthurian legends is still "The Once and Future King" by T.H. White.