Crossword Genius

Engineers abandoning a derelict broken-down castle? (7)

Ross

I believe the answer is:

citadel

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'castle?' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

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(Other definitions for citadel that I've seen before include "Keep - castle" , "Stronghold usually within the walls" , "Stronghold into which people could go for shelter during a battle" , "Bastion" , "Kremlin" .)

I've seen this clue in The Telegraph.
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