Maybe it could be explained ,that , like a lot of monuments it has a history ,with buildings being built over earlier buildings , others collapsing etc .The hope /expectation seems to be that the earliest monument was a chambered cairn .
The earliest dates so far are c.3512 BC (and they are still digging ) and the first enclosing wall is believed to be about 500 years later , so a fortification description is not likely to be applicable at that point . Contemporary Skara Bare and Barnhouse have nothing suggestive of of fortification and although we know the period probably had plenty of inter personal violence there is no sign of any warfare or weaponry as was obvious at some CE 'S .