Britain (or, part of it, anyway), c.625 CE.
ITTL, King Arthur was real, born c.590 CE.
Oh, yes, Arthur was a real man, and was the first-born son of Uther I (b.561, r.580) of the House of Pendragon, King of Carlisle/Camelot and Mediator of the Britons Confederacy- a pact formed for mutual protection when the Romans up and left. Uther was overthrown and assassinated, however, in 594 CE, when he claimed the pretentious title
King of the Britons, which offended the latent yet influential Anglo-Saxons of the east coast.
His domain fell into civil war, while the Saxons swept into southern Britain and utterly ravaged the confederation he had fought long and hard to mediate. The Saxons turned tribe against tribe, until they held dominion over almost all of southern and eastern England. Arthur, son of Uther, was been hidden away since he was a boy, and managed to execute a coup d'etat against his cousin, who had usurped the throne of Carlisle, and seized power in 610. Within 15 years, he managed to lead armies to war and unite Northwest England under the Crown of Pendragon, claiming the title
King of Camelot, of Manchester, of Leeds, of Gwynedd, and of Avalon. However, he refrained from claiming the throne of all the Britons, but forged a new Briton Confederacy, with Trenton, Dyfed, North and South Severn, Glywysing, Summerset, Dorset, and Cornwall, and made powerful allies with the Empire of Ireland, Pictia, and Strathclyde. As November of 625 crept up upon Britain, the great offensive of the Celtic Alliance against the many squabbling Saxon kingdoms was prepared to begin.