As I've said in previous posts, one of the concepts of the Exiles books was that people from Celtic and Greek myths are based on time travellers from the 22nd century who go through a one-way time gate to the past,  who then have myths woven about them.

I realised that author Julian May might have been mining through Welsh mythology when I read this from 'The Celtic Lunar Zodiac' by Helena Paterson in the entry about The Hawthorn Tree:

"In Welsh mythology the hawthorn appears as the malevolent giant Ysbaddaden Benkawr [ Marc / Abaddon], the father of Olwen [Cloud Remillard]. Kulhwch, [Kuhal] son of Kilydd [King Thagdal], seeks to woo and wed Olwen, but the giant hawthorn puts every obstacle in the way of the marriage by demanding a dowry of 13 treasures, all impossible to find. The giant lived in a castle* guarded by nine gatemen and nine watchdogs, revealing the strength of the taboo against marriage in the month of the hawthorn."  Olwen is also known as the Summer Flower Maiden, which also suits Cloud Remillard.  Marc was 6"5 tall which he resented bitterly growing up - for him, puberty was a big step towards the Dark Side.

*Patricia Castellane was the mistress of Marc at the time.

The amazing thing I like about the Exiles / Milieu books is that when you think you've found all the layers, nope, there's still more to find.