Acceptable.
Decent game, I played all the way through until I earned Gold on all castles.
Started off too easy with immediate access to multi-shot. There's absolutely no reason to ever use the single shot. Wasn't until about the third set of castles that the latest level of multi-shot wasn't an instant win. The final set of castles was nice and difficult, frustratingly so at times, but still doable. Then the single large bomb earned for completing all 50 castles just made it way too easy to go back and get gold on everything. Multi-bombs was just overkill.
I wouldn't normally grade this low just for the game-play - I also found technical fault with the game. My computer normally runs at 32-34 C surfing the web, or playing Majong. When playing games like Fallout 3 or World of Warcraft (all on maximum settings), my computer still only gets to about 45-50 C. Running Crush the Castle put my core temperatures at 62 C, even with the auxiliary fan running at maximum. There is no reason for a flash game to generate that kind of strain, especially when it's a 2D game.