... that I'd mistaken for more than one fortress (8)

I believe the answer is:

citadels

I'm a little stuck... Click here to teach me more about this clue!

'more than one fortress' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for citadels that I've seen before include "Fortresses overlooking cities" , "Dials etc up for fortresses" , "Forts in cities for protection and refuge" , "Keeps" , "Strongholds for shelter during battles" .)

I've seen this clue in The Times.

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