I had always assumed it was total fiction but recently heard some people believe there was a King Arthur, or perhaps the story is based on an actual King. Obviously, I think the sword in the stone or lady in the water are fiction, but in your opinion is there any truth to the round table, or bringing peace to England?
Guy Halsall actually does a fantastic job of showing that there is almost no evidence for the existence of a historical King Arthur in his book "Worlds of Arthur" and he does so with a great command of the actual scholarship, rather than the cherry picked second hand claims of more popular "Arthurian" historians.
In a nutshell, there "may" have been a historical person from which the King Arthur legend is based, but there is certainly little evidence for him. And unless some turns up, there is not much we can do to clarify his existence.