Hi Paul,
I had a look on the Magic map
http://magic.defra.gov.uk/
(I searched for Crowden and then picked designations - land based - historic statutory - scheduled monuments off the menu at the left)
and that comes up with 'a roman fortlet', in fact it says "The Roman fortlet 320m east of Highstones is a particularly rare and well
preserved example of this type of monument in Derbyshire." - is that the one you mean?
http://magic.defra.gov.uk/ Metadata_for_magic/rsm/29972.pdf
another good place to look for things is through the Heritage Gateway
http://www.heritagegateway.org.uk/gateway/chr/