Steve Blake and Scott Lloyd put a great case forward for where Avalon is in their book The Keys to Avalon. Not a great book, and they themselves aren't too happy with what the publishers did to it, but it theory on Avalon is a good one.

First we need to go back to the original name for Avalon: Afallach in modern Welsh and probably Abaloch, Avaloch (the ch pronounced as in the Germam achtung) in Early Welsh. Afallach was thought to be a North Walian god, but Blake and Llyod's theory is that he was a real person. They went in search of sites in North Wales that might be associated with the man/god and found an ancient hill fort in northeast Flintshire called Caerfallwch. Knowing how Welsh words mutate, this indeed could have been Caer Afallach. This got them excited, but they then had to wonder about the 'Ynys' (island) part of the name. Their research lead them to discover, and I've check all this myself, that 'ynys' could mean more than island in Old Welsh (and indeed in modern), it can mean 'peninsular' and 'enclose land'.

Ok, what about the sacred waters part? Well, bordering this area to the east is the sacred river of the Dee.

Ok, what about the 'Annwn', 'Land of the Dead' bit? Well, this site sits right in the middle of one of the larges concentrations of Bronze Age to 'Dark Age' burial sites in Europe, with over 3000 known sites and certainly burials from the exact era of Arthur. Even up until recent times those in South Wales referred to the nprth as the Otherworld.

Whilst the Benedictine monks of Glastonbury were saying they'd found Arthur's bones - which just happens to have occurred after a fire at the abbey for which they needed vast sums of money to repair and what better way to get it than finding the bones of a mythical king in order to get pilgrims and money... and it worked - the Cistercian monks of North Wales wrote about Arthur's battle at Camlan and how he asked to be taken back to Gwenydd to be healed or die, because that's where he was from and that's where Afallch was! The monks didn't say they'd found his bones at their abbey, or even exactly where Afallch was, so they didn't gain anything by it.

They also have some very interesting theories on Glastonbury, or rather, Glasteningburgh, but you'll have to read the book find out more.