Silbury,Silbaby, Marlboro - none known to be certainly tiered originally, only one currently proved to be prehistoric, so Egyptian parallels are a bit premature, tho' I'm not against the idea.

We need a sign that the three are truly of similar nature. Spring fed moats is certainly exciting, as it might apply to all three, which is what makes Silbaby so exciting - three is so much more significant than two.

I revisited Silbaby yesterday. You CAN see both Silbury AND the Sanctuary from on top of it (exactly 180 degrees apart to left and right) and facing you to the front is a perfect view of WKLB, in full profile, not the half hidden view that's apparent from the surrounding lowland. So. for what it's worth, Silbaby is in a stonking position. Is it a "significant" position? I don't know, but it can be described in these terms - any megarak allowed to build a house round there would definitely choose the top of Silbaby....

One more thing that might be highly significant - Silbury, in plan, is octagonal. I THINK Silbaby is too. I'd be glad for others to go there and decide if I'm being fanciful or optimistic.