Nice work. The enemy sprites had a few glitches w/ compressing/turning weirdly, but nothing major. I would recommend having the background music loop over all the levels instead of starting over in each level, though. (I'd also recommend programming the game to quit on ESC or have a "quit" button!)

Really liked that this was a stealth game.  Great job.

Simple and straightforward. 

Audio design recommendation - your song's duration is longer than the gameplay of your levels. This means that I don't ever get to hear the full song, and I hear the same part of the song at every level start. Consider allowing your music to persist between levels when this is the case.