Last guard for castle building (4)
I believe the answer is:
keep
I believe this is a triple definition!
'last' is the first definition.
(thesaurus)
'guard' is the second definition.
(keeping guard is a kind of guarding)
'for' acts as a link.
'castle building' is the third definition.
'keep' can be an answer for 'castle' (keep is a type of castle). I am not certain of the 'building' bit.
Can you help me to learn more?
(Other definitions for keep that I've seen before include "Castle tower; cost of maintenance" , "Board and lodgings, perhaps" , "Subsistence" , "Retain in old castle (4)" , "Continue" .)