How to solve castling effect?

[Chazar]

Actually I like the way sieging and storming is handled. Maybe one should simply prolong the time & money needed to built a castle - and possibly offset this by intermediate states of the castle (i.e. some fortress has 3 building stage, where it already offers some minor protection). I think this would also be more realisitic, as castle took years to complete in reality anyway.

Just my thoughts though, since I havent encountered "castling" yet and have a hard time to imagine that this is really a useful tactic. Afterall the attacker benefits from a castle as well, so I imagine that it merely disables raids rather than borderline wars...