Aber Seint and Caer Aber Seint

Caer Aber Seint was originally the Roman fort Segontium, at the mouth of the Menai straits, opposite the Isle of Anglesey. The original structure--probably built by Julius Agricola against the offensive manuvers of the Ordovices tribe--dates from about 77 CE, and was named for the river Seoint. It is thought to have been occupied until the end of the fourth century. Later, Caernarfon Castle was built near its site.

There is some speculation that the castle mentioned in Breudwyt Macsen refers specifically to Caer Seint; D.D.R. Owens believed that it was the original grail castle.

SOURCES:

"Castles and Defensive Structures in Wales and the Borders." WWW: Celtic Mist. nd. URL: http://www.celticmist.freeserve.co.uk/castles.htm

Owen, D. D. R. The Evolution of the Grail Legend. London: University Court of the University of St. Andrew, 1968.

Thomas, Jeffrey L. "Caernarvon Castle" WWW: Castles of Wales. June 27, 2003. URL: http://www.castlewales.com/caernarf.html.