Amazingly well done
My only gripe is that most of the assassination is waiting for the guards to quit camping the exit point. A few ideas to make the game more challenging: You could make it so that when the target sees you, he runs away, maybe not quickly, but some kind of escape, and two, you could implement a sound system, where your footfalls make noise, and you hold a key to walk slowly, which softens the footsteps. You could also put in some kind of guard disabler, like a stun, which you could only use on one guard at a time. In level 8, there were 3 guards that kept coming at me right off the bat, so taking one out for a few seconds would have really helped.
All in all, a really great game.