Castles in the air? (5)

I believe the answer is:

rooks

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'castles' is the definition.
(I know this)

I cannot understand how the remainder of the clue works.

(Other definitions for rooks that I've seen before include "Chess castles and birds" , "Overcharges" , "Large, crow-like birds" , "Chess pieces or black birds" , "Collective name for them is a parliament" .)

I've seen this clue in The Guardian and The Globe and Mail.

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