Preserve part of castle (4)
I believe the answer is:
keep
I believe this is a double definition.
'preserve' is the first definition.
('keep' can be a synonym of 'preserve')
'part of castle' is the second definition.
('keep' is part of 'castle')
(Other definitions for keep that I've seen before include "Retain; strong tower" , "Board and lodging" , "Retain or care for" , "Retain; part of castle" , "Retain, maintain" .)