Visually, this shooter is way ahead of it's contemporaries from the time. What many graphic designers are coming around to more and more as time goes on is that simplicity in design can go just as far if not farther than complexity. While there isn't too much here to comment on, what's here looks sleek - the buildings consist of walls and windows, but really that all it needs in order to provide the feeling of depth like you did. In addition, the enemies are well animated enough that I have a hard time figuring out whether these are PNGs or original animations.
The rest of the package...well I can't skip over it can I. Let's start with sounds. Or not because these are some of the most gut wrenching "gun" sounds I have ever heard. They are so beyond grating to listen to. Just play with no sound. Combat is fun, but very quickly becomes overwhelming to the point of being unable to keep up. Now a good arcade game will get to that point, but over time. Within like 15 seconds you'll be fighting to stay alive, and losing that fight.
All in all, it's a nice start to the package but could have been so much better with some extra polish.