Act as custodian of castle (4)
I believe the answer is:
keep
I believe this is a double definition.
'act as custodian of' is the first definition.
(I've seen this before)
'castle' is the second definition.
(keep is a type of castle)
(Other definitions for keep that I've seen before include "Living" , "Retain, maintain" , "Save" , "Retain; part of castle" , "Subsistence" .)