Stone central to the building of castles? (5)

I believe the answer is:

slate

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'stone' is the definition.
(slate is a kind of stone)

I can't explain the rest of the clue.

(Other definitions for slate that I've seen before include "A roof-tile; criticise heavily" , "Grey metamorphic rock" , "Roofing material or credit recorder" , "Roofing tile" , "Metamorphic rock that can be split into thin layers" .)

I've seen this clue in the Evening Standard.

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