Hey! First off, really nice art and animations! Some of the best I've seen in a bit ;) Keep it up! However, I do have 2 suggestions concerning gameplay:
1. Being able to aim and shoot with the mouse would be absolutely awesome!
2. Bullets probably shouldn't be able to go trough walls :/
Now, the thing with those 2 suggestions is that you can implement one without the other, so that the game will still be improved(imo) while still keeping a bit of difficulty. You could also implement both but that would make a game a lot easier (as well as a lot more user-friendly/fun), but it's up to you!
Good luck for continued development!
P.S There are only 2 animations available at the moment, right?