Outside the entrance to the passage at Bryn Celli Ddu is the site of an Ox Burial. This tends to get overlooked in the excitement of exploring the site's other features, but in 2002 I dowsed it, and was surprised to pick up a rectangular labyrinth-like movement that enclosed it before heading into the entrance of the chamber's passage.
I've just found some interesting information in Mysteries of the Ancient World, Edited by Judith Flanders, [Weidenfield & Nicolson, 1998] under a chapter about the Druids. (And we hear that there was much evidence of them in Anglesey!) This concerned a site in Northern France, called Gournay-sure-Aronde. Martyn has just put up a site page for it, and I've added some comments and a full English webpage info piece about the site.
I will add the info in the book later.
I wonder if King Arthur's Hall on Bodmin Moor was another such ritual area?
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