Since the troops are allowed to be hidden, why not allow the catapults to destroy stuff. Make it hard enought that you can't just launch one catapult to destroy a castle but a little something to encourage the people who hide their troops to expose them. Perhaps each building has a point value. If it is attacked it loses points. This could even introduce another building like a repair shop to fix buildings as they become damaged. If a building takes enough damage it loses a level.