What parts of the King Arthur myth show Celtic culture?
First there is no “King Arthur myth’. There are a number of medieval stories about Arthur, most of them written in French and obviously as much or more influenced by the French chivalric culture of the day than by Celtic influences deriving from the supposed source. There is some difficulty in identifyng Celtic culture. Many Celtic tales share motfis with Germanic tales, and even African tales. And few Celtic tales of Arthur survive. For those that survive, see http://www.ancienttexts.org/library/celt… where you can find the Welsh Arthurian tales: “# Culhwch and Olwen”, “The Dream of Rhonabwy”, “Owain, or, the Lady of the Fountain”, “The History of Peredur”, and “Gereint and Enid”. But the last three for these mostly derive from the French romances of Chrétien de Troyes. And see http://www.maryjones.us/ctexts/trystan.h… for t