This is really well put together! The graphics and the gameplay are both really well thought out, and I only wish there were more levels!
The only bug I found is that you can still undo moves even after getting a game over (and this undoes the 'game over' screen). Doing this sometimes throws the guards positions a step or so out of order, which I was able to use to beat one of the levels. Apart from that, the game was excellently constructed, well done! :D