Yes Windmill hill! There is a Stukeley sketch with a windmill on it. They were everybloody where! :)
Take a look at the original sketch instead of the woodcut
http://aveburytour.mysite.wanadoo-members.co.uk/Silbury/Photos/StukeleySketch.jpg
which shows 2 circular features. They are on the SMR as ploughed out disk barrows.
A History of Wiltshire Vol1 Pt1 P216.
'Disk Barrows, Avebury, Waden hill (SE end) approx 10906836.
In Stukeley's time it showed as a disk barrow partly cut by Roman road. Destroyed.'

No one has ever recorded the cropmarks of these Barrows which is very odd considering that the mounds on the other end of the hill are still very visible when the crop is right.
The Description Stukeley gives say a Druid Barrow which he named all the larger barrows.
Its interesting that he says they removed the top.
I have emailed this to several EH people and have asked how I go about getting this scheduled.
PeteG
ps flip me an email [email protected]