In truth, almost none. The legend is just that, a legend. It is also mostly fictitious.

The first datable mention of an 'Arthur figure is in the 9th century when a Welsh cleric Nennius has been attributed to have written The Historia Brittonum. This writes about a Celtic leader, Arthur, defending Britain against the Anglo-Saxon invaders. The reliability has been called into question by historians over the years and most recently by Thomas Green in 2007. The reliability questions arise from the fact that Arthur reportedly killed almost 1000 men in one battle.

The legend really rose to prominence in the 12th century when Geoffrey of Monmouth wrote about King Arthur in the History of the Kings of Britain. This has been seen as an entirely fictitious account. Geoffrey used inaccurate accounts and stated that this was a list history refound in 1138. It was an instant success and started a trend of reading for pleasure in the "12th century renaissance". From his work came the work of Chretien de Troyes who wrote the Arthurian romances and introduced such themes and characters as the Holy Grail and Lancelot.

There is a possibility that the Arthur legend derived from a Celtic warrior or leader. The Historia Brittonum calls him a 'dux bellorum' or 'miles' which indicates in Latin a military leader or soldier so Arthur could be a made up character of a Celtic warrior. Nevertheless, there was resistance to the Anglo-Saxon invasions, hence the existence of the Celtic parts of Britain like Wales, Cornwall and Scotland.

However, what is true is the impact that the Arthurian Legend had on medieval Britain. To the people of the Middle-ages, Arthur was real. Edward III modelled his Knights of the Garter on the Knights of the Round Table. Tintagel Castle was built to be a new Camelot. Owain Glyndŵr raised the Golden Dragon standard of Arthur's father, Uther Pendragon when the Welsh rebllee against Henry IV in the 15th century. And chivalry in the 14th century was heavily influenced by Arthur's legend. Edward the Black Prince of Wales was heavily influenced by Arthur having probably had the books of Arthur in his library. His own personal brand of chivalry often reflected that exhibited by Arthur in the stories.

Richard Barber wrote about Edward the Black Prince that the legends endure more than any real history. And the Arthur legend endures as a major influence on the Middle-ages in Britain and as a story of a Celtic past. No he was not real, but the influence and enduring endearment certainly are.