Sort of Fun...
It was a very fun game, but I ended up being unable to beat one of the levels, I could kill everyone in the castle except the king and queen. The middle of the castle was too heavy to knock down, but I know if I would've knocked it over it would've killed the king and queen.
I also tried to send smaller, faster rocks over the walls to get them, but they ended up being just as slow as the large group of rocks, so it never really worked. you should tweak the game a bit to see if you can make the single small rocks fly faster and longer, so they'll be able to get over any tough walls.
Oh, you might also try adding the ability to send in ground troops to attack them, and try making the people in the castles run away from you.
I hoped this helped,
~Sectus