The wonderful Aubrey Burl, in his must-have book ‘Megalithic Brittany’, says on page 92:
“Tradition has it that each year in ancient times a young girl was sacrificed on the mound on an altar specially built for the occasion. The meaning of Chateau Bû is obscure, but Bû may derive from the Breton word for cattle, bulls having an important role in the religious celebrations of Brittany.”
... as indeed they have throughout the rest of the world, Aubrey.