I agree. It could have some type of catapult, trebuchet, ballista, rams or the likes.
I think having sets where one can built carts and wagons, or various medieval like boats or small fortresses would be nice as well. You could have a small building set with houses, small towers, walls, agtehouses, manors houses etc. REally there is alot of things that could be done for a creator for the castle theme.