King Arthur Lived in Brittany

A class of wandering minstrels, with histrionic talents, found that this new material captivated barons and their ladies, not only in Brittany but wherever French was understood. More and more they adapted fantastic tales to French tastes...and introduced the pageantry of chivalry. Their audiences...were fascinated by the new and various tales of love and marvel and adventure, and were more easily persuaded to accept the Breton image of Arthur as the nonpareil of kings. (Roger Loomis)

Whichever way you look at it King Arthur has Breton connexions. 

Either as the real historical figure: the soldier-hero, behind the myth. 
This historical Arthur could, some say, be Riothamus ('the greatest king') a 5th century 'king of the Britons' who led an army into Gaul and whose last recorded position was near the Burgundian town of Avallon. 

This identification has appealed to experts such as Leon Fleuriot and Geoffrey Ashe. Others are fond of spotting Arthurian locations in Brittany, such as the Enchanted Forest of Broceliande (Paimpont, south-west of Rennes) where it is said you can find Merlin's Tomb and the (lady of the) Lake at Concoret. Some even place the island of Avalon at Mont St. Michel. 

Or King Arthur could be seen as the end result of a series of romances, stories and legends told by bands of Breton story-tellers.
King Arthur would have remained an obscure local legend had his cause not been taken up and embellished by Breton bards ('jongleurs') and then taken back across the Channel with William the Conqueror. 
Geoffrey of Monmouth, whose  Historia Regum Britanniae ('History of the Kings of Britain') started the whole Arthurian cycle going, was Breton in origin and not Welsh, as is commonly thought. 

Geoffrey of Monmouth

It was the Breton conteurs, then, who breathed life into Arthur by fusing these distant memories of a British soldier-king with a deeper, darker tradition of Celtic mythmaking.