Leaving aside the romantic embellishments of the Renaissance onwards, it is generally believed that there was an actual Arthur.
The great figure in the struggle between the British and the Saxons is Arthur. He may have been of the Roman tradition, since the Romans had an officer - the Dux Britanniarum (Duke of Britain) - who was leader of a mobile force, charged with the duty of protecting the integrity of the Roman province.
Places commemorating Arthur may be found in widely separated parts of Britain, which suggests that he held such an office. His greatest victory came in about AD 496 at Mons Badonicus, a place perhaps in Sussex or possibly near Bath. The victory halted the Saxon advance for at least half a century.
Whether Arthur was actually a king or merely a warrior, however, is less clear. It is believed that he was the leader of Brythonic forces, and was killed following the battle of Camlann in about 515.
In the earliest mentions of Arthur in Welsh texts, he is never given the title of king. Medieval Welsh texts frequently referred to him as ameraudur (emperor or war leader). There is now a broad consensus that he was a Romano-British warrior leader defending Britain against Anglo Saxon invaders in the late fifth or early sixth century.