Possibly a UFO landing site! (lol)

or,

http://www.themodernantiquarian.com/site/304/dragon_hill.html#links
(Dragon hill maybe had a building on the top)

http://www.telegraph.co.uk/gardening/4385933/Silbury-Hill-mystery-soon-to-be-resolved.html
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Archaeologists found a series of medieval pot-holes on top of the hill, indicating a large building. The discovery of two arrowheads also suggested it had a defensive purpose in the period of the Danish invasions early in the 11th century, or around the time of the Norman Conquest. There is speculation, too, that Silbury was originally dome-shaped in its prehistoric form, and that its current flat-topped aspect was the result of later lopping off to create its military function.