El Cid at ruined fortress (7)
I believe the answer is:
citadel
'fortress' is the definition.
(citadel can mean a fortified city)
'el cid at ruined' is the wordplay.
'ruined' indicates anagramming the letters.
'el'+'cid'+'at'='elcidat'
'elcidat' is an anagram of 'CITADEL'.
(Other definitions for citadel that I've seen before include "Bastion" , "secure place" , "Stronghold (of the Salvation Army?)" , "Fortress -- dialect (anag)" , "Keep - castle" .)