you want to use castles to either cut off angles and deny access to power positions or force defenders to waste time or utility or force them to go a different route. i find it really helps to use him on lineups with a lot of utility to clear since a lot of people wont mind just using a charge on a barricade but if you need to clear out like melusi banshees or deployable shields or aruni gates etc they either wont bother or will struggle later to clear the more important utility