hi! there's lots of room for more input here, but there have been a few posts on Stonehenge that you may find interesting

• How was Stonehenge historically viewed? - overview

• What did the Romans think of Stonehenge? - overview

• What, if anything, did the Romans make of the megalithic sites they came across during their conquests? - Diodorus Siculus

• When is Stonehenge first mentioned in history? Did the Romans mention it? - Geoffrey of Monmouth

• What theories were there about Stonehenge in medieval times? - comment on Geoffrey of Monmouth

• What were the historical attitudes towards structures like Stonehenge? - more on Geoffrey, and other miscellany

• What has stopped Stonehenge from being seriously vandalised during its thousand of years lifetime?

and a couple of brief comments on Hadrian's Wall

• Did the military entities that occupied Britain after the Romans withdrew think of Hadrian's Wall?

Just the thought of all these mixed buildings and landmarks from Roman towns to round houses and hill forts crumbling away must have confused people for centuries after.