Destroy what's left of castle (4)

I believe the answer is:

ruin

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'destroy' is the definition.
(I know that ruin is a more specific form of the action desolate)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for ruin that I've seen before include "failure" , "Wreck" , "Physical collapse" , "Destroy or bankrupt" , "Destroy; downfall" .)

I've seen this clue in The Mirror.

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