Hold on to the castle (4)
I believe the answer is:
keep
I believe this clue is a double definition.
'hold on to' is the first definition.
('keep' can be a synonym of 'hold on to')
'the castle' is the second definition.
(thesaurus)
(Other definitions for keep that I've seen before include "run" , "Fort" , "Retain in old castle (4)" , "Retain, maintain" , "Hold - fortress" .)