It is the Sanctuary.

It is easy to be misled by this as there is an unstated agenda. Look to his difference with Lubbock over Silbury ahd his corrupting certain names to fit (rather put to the sword by A.C. Smith).

He read Bay Down, as corrupted from Rae Down, as Badon as in Mount Badon, and made Silbury the burial mound of the Arthur's battle of Mons Badonicus.