Here is what is known with certainty about the historical fact behind the Arthurian mythos:
  1. There lived a prince called Tristan.
Here are what are probably historical facts behind the Arthurian mythos:
  1. Around the year 500 a Christian Romano-Celtic war leader called Arthur defeated the incoming Saxons in a series of battles, culminating in one at a place called Mount Badon.
  2. After about twenty years of peace, Arthur fell in battle with Mordred, at a place called Camlann Field.
Here is the more reliable contemporary evidence for the above:
  1. The gravestone of one Drystan exists. It is of about the right age. The name Drystan became popular in that period. This implies that he was a prince whose exploits earned him renown.
  2. The monk Gildas writing in the middle of the sixth century, in a sermon, reminds his audience of the anarchy and disorder that prevailed many years ago, and how the heathens were defeated and peace was brought. He says that the final battle happened on the very day when he, Gildas, was born, forty-four years before. He does not mention Arthur by name. He does mention Aurelius Ambrosianus. But he was speaking to an audience some of whom would have been alive at the time of the events he describes, so he probably had to be fairly accurate.
  3. The Welsh Easter Annals (written by monks in Latin) have two entries mentioning Arthur. The annals were recorded at the time, and are a reasonable historical record.
    • One entry says that he fought battles, the last of which was at Mons Badonicus; and that he bore the cross of Christ upon his shoulders. Later fables had him going in lugging a huge life-size crucifix, but fairly obviously it could just mean that he wore a cross emblem on his clothes. Or it could be a confusion between shield and shoulder, very similar words in Old Welsh.
    • The other entry, dated about twenty years later, says that he fell fighting with Medraut at Camlann Field. (I can't remember the precise spelling of either name, but the point is they both do occur.)
      Note that strictly speaking the original Latin is as ambiguous as the English: 'fighting with' could conceivably mean on the same side as; but we can assume later legends of their fighting against each other are correct.
    • The annals were copied out periodically by the monks to preserve them; unfortunately the copies we actually have are considerably later than the events, and by this time the growing legend of Arthur might have led later copyists to insert material that was not in the original. The passage about carrying the cross is particularly suspect.
    • They were written just before Bede popularized the Anno Domini system, so the dates are according to Roman dating, but with a hitch: it is not clear which of two systems was used, so the base date could be one of two twenty years apart. One possibility is that Badon was in 516 and Camlann in 537.
Here is the evidence which is a couple of centuries later, so its evidential value has been contaminated by what was clearly a rapidly growing legend of the great Arthur:
  1. The monk-historian Nennius (Welsh Nynniaw, century unknown, perhaps 800s) says he had heard that Arthur was a British warrior who defeated the Saxons, chosen twelve times to be commander by all the kings of Britain, and he fought twelve battles, of which Mount Badon was the last.
    He also says his grave varied in size, and that he himself tested this, and found it to be true.
    Last time I looked for this statement I could no longer find where I had found it; I suspected it was somewhere in Charles Williams; or possibly C.S. Lewis; but I could not find it. Or perhaps it had just shrunk. Could you please /msg me if you know where this story comes from.
    Ah, thanks to aneurin for pointing out this story is in Nennius but refers to Arthur's son Arw. The full text of Nennius is at www.fordham.edu/halsall/basis/nennius-full.html
  2. Several old Welsh poems recount his exploits; but the manuscripts of these are mediaeval, so who knows what insertions were later made; and none of the three greatest early bards, Taliesin, Aneurin, or Llywarch Hên, mention him: they praise Urien lord of Rheged as best of warrior-kings.
Everything else is mere romance, more than half a millennium after the true events. It is impossible to reliably extract historical inferences from Wace, Layamon, Geoffrey of Monmouth, or Chrétien de Troyes. We know nothing really true about Lancelot, Guinevere, Merlin, Galahad, or Excalibur, except that Arthur was a great enough real-life figure to inspire it all.
King Arthur, or the British Worthy is the title of a semi-opera by Henry Purcell, libretto by Dryden, produced in 1691. It features the song Fairest Isle (name of the BBC tercentenary celebration year), and at one point a teeth-chattering chorus of people frozen by the Cold Genius.