castle barriers (5)
I believe the answer is:
moats
'castle barriers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moats that I've seen before include "Ditches around castles" , "Trenches around castles" , "Water defences" , "what isolate castles" , "Water-filled ditches as fortifications" .)