Firstly, it's worth making the point that the King Arthur of legend almost certainly did not exist. There may have been a 6th century chieftain upon whom some of the legends are based.
Geoffrey Ashe in "In Search of Arthur's Britain" details excavations at South Cadbury Castle, near to modern day Queen Camel. These show the early Christian stronghold of such a man. Perhaps the man was Artorius (the Romano-Celtic form of Arthur. Curiously, while there is no historical evidence for Arthur, there is a little for three of his knights. Gawain, Tristram and King Mark are almost certainly "real people", dark ages figures who left a little imprint on history.