The game has an outstanding level of polish. The graphics and audio fit together really well. There are some times where the audio was too screechy sounding but it wasn't too much of an issue and just a nitpick. 5 stars for graphics and sound, no problem there.
The fact that you can do more damage to enemies by shooting them in the back is a clever idea, and the way you introduced it via level design was something I caught on to on my second playthrough. That was a good thing because I learned how to manage my bullet resources more effectively. However I would have liked the 'shooting in the back' mechanic it do be explored much further in terms of strategy, maybe in finding a way to lure enemies to a trap?
The game could have been more fun to play if the movement wasn't too slow. I almost died at my first attempt at fighting the final boss but I beat him with just a sliver of health left. The only reason I won was because I ran away and grabbed some health I didn't pick up earlier, and if I died and started all over, I would have quit right then if I had to slowly crawl back to the end.