Custodians in castle by royal square (7)

I believe the answer is:

keepers

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

'castle by royal square' is the wordplay.
'castle' becomes 'keep' (I've seen this in other clues).
'by royal' becomes 'er' (abbreviation for Elizabeth Regina. I am not sure about the 'by' bit.).
'square' becomes 's' (abbreviation).
'keep'+'er'+'s'='KEEPERS'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for keepers that I've seen before include "Players in goal" , "Goalies" , "Zoo attendants" , "Custodians, stewards" .)

I've seen this clue in the Sydney Morning Herald.

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