There's a way to solve it without exiting the castle. Just push the top cannon down. (Perhaps I shouldn't have made that exit so easy to find... Then again, there is an easter egg outside one or two of the castles...) It's really my first puzzle game, and it was sort of a tradeoff between puzzle difficulty and the time I had left to finish the game! Of course, it's a bit harder without Undo, Reset, or the secret Cannon Pull maneuver, but it's possible!
The last level is actually based on Bodiam Castle, built by Sir Edward Dalyngrigge. I hear he was well paid by Richard II to fortify it against French invasion. So I tried to put that story into my game. ;)
Thanks for your comments!