King Arthur: a myth or an actual post Roman chieftain ruling in an era when the written record fell out of fashion. Who knows?
Clearly something was going on; so-called Offa's Dyke turns out to be centuries older than Offa, for example. The Wansdyke probably postdates the Roman withdrawal, and could mark the southern frontier of a good-sized Romano-British realm. The old Hadrian's Wall and its forts would mark the north, and the Cardyke -- the Roman canal -- probably was at or near the eastern frontier. The *name* Camelot seems to be a corruption of Camulodunum (Colchester). Since the first Roman capital was there, the legend of Arthur is probably a pastiche of lots of unrelated snippets from many places and eras. Whoever it was that is the primary character that has become the folkloric Arthur (and assuming there is one), probably had his capital at Ely, Cambridgeshire.