For more than a thousand years different races, different ages, had taken hold of the ancient Celtic legends and spiritualised them after their own manner, and moulded them to their own ideals. There may have been a historical Arthur, Comes Britanniae, after the Roman withdrawal. Ye Amherawdyr Arthur, "the Emperor Arthur," may have lived and fought, and led the Brythons to battle. But there may also have been a Brythonic deity, or culture hero, of the same, or of a similar name, and myths about him may have been assigned to a real Arthur. Again, the Arthur of the old Welsh legends was by no means the blameless king--even in comparatively late French romances he is not blameless. But the process of idealising him went on: still incomplete in Malory's compilation, where he is often rather otiose and far from royal. Tennyson, for his purpose, completed the idealisation.
As to Guinevere, she was not idealised in the old Welsh rhyme -"Guinevere, Giant Ogurvan's daughter, Naughty young, more naughty later."Of Lancelot, and her passion for him, the old Welsh has nothing to say. Probably Chretien de Troyes, by a happy blunder or misconception, gave Lancelot his love and his pre-eminent part.
Lancelot was confused with Peredur, and Guinevere with the lady of whom Peredur was in quest. The Elaine who becomes by Lancelot the mother of Galahad "was Lancelot's rightful consort, as one recognises in her name that of Elen, the Empress, whom the story of Peredur"(Lancelot, by the confusion) "gives that hero to wife." The second Elaine, the maid of Astolat, is another refraction from the original Elen. As to the Grail, it may be a Christianised rendering of one or another of the magical and mystic caldrons of Welsh or Irish legend.
There is even an apparent Celtic source of the mysterious fisher king of the Grail romance.
A sketch of the evolution of the Arthurian legends might run thus:-Sixth to eighth century, growth of myth about an Arthur, real, or supposed to be real.
Tenth century, the Duchies of Normandy and Brittany are in close relations; by the eleventh century Normans know Celtic Arthurian stories.
After, 1066, Normans in contact with the Celtic peoples of this island are in touch with the Arthur tales.
1130-1145, works on Arthurian matter by Geoffrey of Monmouth.
1155, Wace's French translation of Geoffrey.
1150-1182, Chretien de Troyes writes poems on Arthurian topics.
French prose romances on Arthur, from, say, 1180 to 1250. Those romances reach Wales, and modify, in translations, the original Welsh legends, or, in part, supplant them.
Amplifications and recastings are numerous. In 1485 Caxton publishes Malory's selections from French and English sources, the whole being Tennyson's main source, Le Mort d'Arthur.
Thus the Arthur stories, originally Celtic, originally a mass of semi-pagan legend, myth, and marchen, have been retold and rehandled by Norman, Englishman, and Frenchman, taking on new hues, expressing new ideals--religious, chivalrous, and moral. Any poet may work his will on them, and Tennyson's will was to retain the chivalrous courtesy, generosity, love, and asceticism, while dimly or brightly veiling or illuminating them with his own ideals. After so many processes, from folk-tale to modern idyll, the Arthurian world could not be real, and real it is not. Camelot lies "out of space, out of time," though the colouring is mainly that of the later chivalry, and "the gleam" on the hues is partly derived from Celtic fancy of various dates, and is partly Tennysonian.
As the Idylls were finally arranged, the first, The Coming of Arthur, is a remarkable proof of Tennyson's ingenuity in construction. Tales about the birth of Arthur varied. In Malory, Uther Pendragon, the Bretwalda (in later phrase) of Britain, besieges the Duke of Tintagil, who has a fair wife, Ygerne, in another castle. Merlin magically puts on Uther the shape of Ygerne's husband, and as her husband she receives him. On that night Arthur is begotten by Uther, and the Duke of Tintagil, his mother's husband, is slain in a sortie.