Maintain castle tower when built (4,2)
I believe the answer is:
keep up
'maintain' is the definition.
(I know that maintain can be written as keep up)
'castle tower when built' is the wordplay.
'castle tower' becomes 'keep' (I've seen this before).
'when built' becomes 'up' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'keep'+'up'='KEEP UP'
Can you help me to learn more?
(Other definitions for keep up that I've seen before include "Stick with" , "' - - with the Joneses'" , "Stay in contention" , "Match pace or store aloft" , "Continue to be in touch (with)" .)