"It seems that there was a war leader, whose name we do not know, who defeated the Saxons, checking their advance temporarily ... Eventually the name Arthur adhered to this folk memory, and his list of accomplishments grew. Arthur is in many ways greater because we do not know the truth ..." Source: Britain Express.
"The real Arthur may have been a man named Ambrosius Aurelianus, or perhaps his war leader, who defeated the Saxons in a major battle we know as Mount Badon ... halting their advance for as long as forty years. In the end, however, the superior might and numbers of the Saxons and their allies were too much for the islanders, and Arthur's efforts became little more than a historical footnote. A terrifically romantic and exciting footnote though, for Arthur and his deeds were woven like a silk thread into the fabric of myth and legend in which Celtic storytellers delight."
Did a King Arthur actually exist? As mentioned above, various attempts have been made to connect the legendary king with actual historical figures. For instance, according to a 1998 Archaeology story, "possible evidence of the existence of Arthur, the legendary warrior king, has been found at Tintagel in Cornwall. A Cornish slate with sixth-century engravings was found in July on the eastern terraces of Tintagel on the edge of a cliff overlooking the place traditionally known as Merlin's Cave [see next video for more on Merlin]. It was discovered under broken pottery and glass from the late sixth or seventh centuries during the re-excavations of an area last dug in the 1930s." The evidence here consists of a Latin inscription which may refer to Arthur.
This next video shows the ruins of Tintagel Castle: King Arthur's birthplace, according to Geoffrey of Monmouth.
Tintagel is only one of the Arthurian locations proposed over the years. Historic UK has a list of places that have been associated with Camelot, though the consensus is that the legends are entirely fictional. That could be true, but these stories must have come from somewhere. "The King Arthur we know is one of romance, ephemera and myth," according to the Independent (2010). "But is he real? Arthur has been in and out of fashion more than denim: one year his veracity is being argued by every archaeologist in Britain, the next he's ignored or derided."
Video from 2016.
Maybe I'm just a hopeless Romantic, but I like to think that King Arthur actually existed. Despite its implausibility there's a fair amount of documentary evidence to support the idea:
"One school of thought, citing entries in the Historia Brittonum (History of the Britons) and Annales Cambriae (Welsh Annals), sees Arthur as a genuine historical figure, a Romano-British leader who fought against the invading Anglo-Saxons some time in the late 5th to early 6th century." (Wikipedia)
Librovox recording of the History of the Britons by Nennius, read by various.
"The Historia Brittonum, a 9th-century Latin historical compilation attributed in some late manuscripts to a Welsh cleric called Nennius, contains the first datable mention of King Arthur, listing twelve battles that Arthur fought. These culminate in the Battle of Badon [next video], where he is said to have single-handedly killed 960 men. Recent studies, however, question the reliability of the Historia Brittonum."
I really doubt if King Arthur killed 960 men in battle, but it would be interesting to know how Nennius came up with that specific number, assuming, that is, that the 9th century Welsh monk actually wrote the Historia Brittonum. Some authorities believe he compiled the book from various older sources.
"The other text that seems to support the case for Arthur's historical existence is the 10th-century Annales Cambriae, which also link Arthur with the Battle of Badon. The Annales date this battle to 516–518, and also mention the Battle of Camlann, in which Arthur and Medraut (Mordred) were both killed, dated to 537–539. These details have often been used to bolster confidence in the Historia's account and to confirm that Arthur really did fight at Badon."
The most well-known account of the Arthurian legend is Le Morte d'Arthur, first published in 1485 and written by Sir Thomas Malory, whoever he was. Several candidates with the same name exist, but he was most likely Thomas Malory of Newbold Revel, a soldier, former MP and possible "thief, bandit, kidnapper, and rapist," said to have written the book using French and English sources while he was being held prisoner during the Wars of the Roses.
Related: Arthurian Timeline