Charge defensive structure, we gather (4)
I believe the answer is:
levy
'charge' is the definition.
(I know that levy is a more specific form of the action charge)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for levy that I've seen before include "Troops raised" , "draft" , "Tax, fee, fine, etc" , "Charge imposed and collected" , "Amount, forces, raised" .)