Fortresses (7)
'fortresses' is the definition.
(a castle is a fortress)
This is all the clue.
(Other definitions for castles that I've seen before include "Keeps" , "English homes?" , "Forts" , "Fortresses" , "Large and stately mansions" .)
(Other definitions for redoubts that I've seen before include "defences" , "Obtrudes into outer fortifications" , "Temporary or supplementary fortifications" .)