I read Geoffrey of Monmouth some years ago. Fun book.
The Welsh versions of Arthur's stories are housed in The Mabinogion. It is a must read, and one
of my all time favorite books.
There is much debate over his origins.
Some say he may have been a god. In the Welsh story of Culhwch and Olwen, he is chasing an enchanted boar; a motif of the ruler of the underworld.
Historically, he may have been a Britain of Celtic descent, or maybe Romano-Celtic descent, that helped keep the Saxons out of Britain after the island lost the protection of Rome in the 6 th Century.
There was some sort of battle at Badon hill that was victorious for the Brits, and may have led to the Golden age of Arthur's reign.
Eventually, the Saxons did make it into Britain, and the Celts were pushed into Wales, Cornwall and Brittany.
The Celts kept the legends alive and maybe that is why he is so much associated with Wales.
It was only with the later literature, such as Mallory, that Arthur evolved into an English (e.g. Saxon), king.