Many historians now believe that King Arthur did, in fact, exist. There was a break in the Danish invasions of England from 500-550 CE; some think that this is when Arthur (or an Arthur-like king) rallied all of Britain to fend off the invasions that plagued his island from 400-600. At any rate, whoever the king was at this time is probably the source of Arthurian legend. Whether any of the other characters actually existed is not clear.