I was aiming at the feel of 90's shooters, hence the 2d images :)
As for there being more- this was originally made for a two week jam, so some features (more weapons, more bad guys, more levels) got cut. If I were to make a bigger version, there would be more of everything.
As for sound... firstly- I know I'd add more if I had time- footsteps, hurt sounds, that kind of thing. Right now I feel the game gives you just enough feedback via sound, but it could be more fleshed out.