Nice retro shooter there. I was hooked more than I expected and went into a small zombie mode for some minutes and didn't even notice some minutes passed.
So the gameplay itself is great and addictive. It's a bit slow but good for now.
The sfx is fitting for the retro style. There was no music but I got quite focused and relaxed in the same time so I didn't feel the absence of it that much. If you add some music you might not want to do someting with too much action because the game itself has a nice, relaxing feel.
The gun recharge could be a bit faster because I run out many times but it was not a big problem.
Overally it's good job and keep it up!