We can be certain King Arthur was not a medieval King with a bevy of knights in suits of armour and a big castle called Camelot. We know very little historically but if he existed, then King A rthur was probably a 5th century warrior chief who protected his peoples from the Saxon invaders for a time. The battle of Camlan is probably connected to King Arthur. But virually everything about King Arthur is conjecture. He could have come from Cornwall, Wales, Southern England, Northern England . Britain abounds with sites which have King Arthur connections.
King Arthur was said to be the son of Uther Pendragon and Ygraine of Cornwall. Arthur is a near mythic figure in Celtic stories such as Culhwch and Olwen. In early chronicles he is presented as a military leader, the dux bellorum. In later romance he is a king and emperor. The Tudor monarchs traced their lineage to King Arthur and used that connection as a justification for their occupation of the English throne.
Irrespective of whether King Arthur was real or mythical, it cannot be denied that King Arthur has been a major influence on literature, from the early Middle Ages to the present day. And although if King Arthur had been a "real" king, he would have lived around the 5th or 6th centuries, it is more as a Middle Ages knight that he is presented in literature. From Malory to Tennyson to T. H. White. The central story is of a noble kingdom of high ideals, whose ideals are undermined by the adultery of Queen Guinevere with Lancelot, the most noble of all the knights. And eventually brought down by the treachery of Mordred. And in spite of this, Arthur's memory lives on.
Historia Brittonum (written c.830AD) says “Arthur fought against them [the Saxon invaders] in those days, together with the kings of the Britons, but he was the leader in battles [dux bellorum].” The rest of the text lists a number of his supposed battles of which the Battle of Badon is the only one that we know from other historical sources (Gildas’s De Excidio Britanniae of c.540AD, which mentions the battle but not Arthur).
The main impetus for Arthurian stories starts with Geoffrey of Monmouth who records Arthur's birth, childhood, ascension to the throne, military conquests, and death. He places King Arthur as living from the late fifth century to 542, when the king was mortally wounded in his last battle. This story became the basis of the Arthurian legend, and was built on by Chrétien de Troyes and Sir Thomas Malory.
Geoffrey completed his History around 1130, using earlier sources such as Gildas, Nennius, The Annales Cambriae and Bede. But none of these makes any subsantial mention of a King Arthur. So where did he get his facts? Geoffrey claimed to have had in his possession a "certain very ancient book written in the British language." However nobody else has used this book, and if it existed, has not been passed down to modern times
Other attempts to fill-out the above concept of King Arthur have focused on trying to localise this Arthur. Arthur has been placed by various researchers as the war-leader in the North of Britain, the South, the Midlands, southern Scotland as well as Wales and Cornwall . But these are not particularly sucessful at localising King Arthur, as the early "historical" references just are too vague to tie him down.
But the main source of Arthurian myths did not come from British English writings. Rather from French authors living in Brittany about 300 years before Malory's epic Arthurian tale was published. They might well have based their tales on stories told by English Crusaders in the 11th century, but they undoubtedly embroidered these stories themselves. These early romantic novelists gave us most of the myths that was later further embroidered by Malory and Tennyson.
So really King Arthur, the 'King of the Britons' was in probably a French invention, as are so many of the details of the legend that come from these early French writers. Lancelot, his affair with Guinevere, Excalibur, Camelot, the Round Table, Sir Perceval all originate with these French writers.
And at roughly the same time Geoffrey of Monmouth was writing his History of the Kings of Britain. In Geoffrey's story Arthur lived during a war-torn period in Welsh history, and becomes the leader of a large kingdom he has created by military victories against the invading Saxons. In Geoffrey's account, Arthur goes on to invade France, defeats the Roman armies and almost conquers the remnants of the Roman empire.
It seems that there was a war leader, whose name we do not know, who defeated the Saxons, checking their advance temporarily. As time went by people remembered this leader fondly, the good old days and a touch of "Oh, if only ... was our leader now". Eventually the name Arthur stuck as his name in folk memory, and his ascribed deeds grew long after his death - King Arthur arrives, the once and future king.