This is a beginning. What is done is good but there is place for improvements.
The gameplay makes the game too boring. You just shoot zombies, like that, forever - and it's way too easy, even in hard mode. It would be better if :
- 1) Pickups spawn less frequently (Hard mode has, like, 10 pickups simultaneously on the screen; I had 100 health and bonus weapon all the time);
- 2) The game was a little harder (or would get harder and harder as time goes) and, mostly,
- 3) The game could be wave-based (it might seem a bit far from the original idea, but I feel it could make the game more fun to play). Not only it would make it easier to progressively increase the difficulty (more/stronger zombies each wave), but it would also allow some variety in the enemies. Some random ideas : You can make zombie that takes several shots to kill, zombies that go faster, or even bosses.
Example : wave 1 starts with a few 1-shot-killed zombies, wave 2 goes with a greater amount of these, then wave 3 introduces stronger zombies, then a few waves later fast zombies are introduced, then the next one mixes all zombie types introduced yet, and each, like, 10 waves, there's a boss.
(Yes, that's classic and ordinary, but I feel it would already help making the game more fun to play. Also, you might have a completely different idea to enhance the game, what I wrote is just an example or a suggestion.)
Art is correct. It's not impressive, but it's not bad. If you choose to follow the example above, you could make a different texture for each zombie type.
Overall, it looks like it started from a good idea but it would need much more development. The current game is basic and get boring way too quickly, since after a few minutes, the player doesn't discover any new challenge. I suggest you add much more mechanics to the game to liven it up a bit :)