Castles (5)
(Other definitions for ice that I've seen before include "It's cold and slippy" , "Finish decorating the Christmas cake" , "such an age" , "Some number" , "Frozen liquid" .)
'castles' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for forts that I've seen before include "Strongholds" , "Castles" , "Defensive structures where troops are stationed" .)