As well as the possibility of the ditch and bank of a henge being a defence against what is enclosed . There is the the case of the stones of a stone circle having the same function e.g. at Tomnaverie excavation showed that the last act of construction was the erection of the outer stones . BTW John Barnatt and Mark Edmonds did a very good paper in the Cambridge Archaeological Journal 2002 on "Places Apart ? Caves and Monuments in Neolithic and earier Bronze Age Britain " . Mostly centred on the Peak District and and Clywd .