Decent, but no challenge
Pretty good take on the classic top-down shooter, but there were a couple of problems... One was the zombies' hitboxes. They seem to be smaller than the zombie's sprites, which is pretty annoying. It does go a little way to counter my next point though...
There's no real difficulty to it! You start off with a pistol that without any upgrades is well able to handle any enemies. That's good for the start, but when you're halfway through the game and you're still facing the same enemies with no change in strength and not much change in numbers, it gets repeditive pretty fast. I didn't feel like I needed to upgrade, I only did it to make the game go a bit faster. I also never needed to use grenades, flares, or the other guns - they might as well not have been there. When you fully upgrade the pistol, it cantake anything out in pretty much one shot, making the others kinda unnecessary.
Also, you have FAR too much health. The zombies barely take any off per hit, and you have 100 hitpoints, with up to 6 medkits which fully heal you. That's 700 health, when the enemies do less than 10 per hit. And that's not counting the stamina upgrade, which I didn't put any points into.
I didn't actually check this when I was playing, but when you use the helicopter to clear out zombies, does that have an effect on how many spawn when you land? The helicopter has too much ammo anyway, you could kill all the zombies and have half your bullets left.
Overall it's good, nice atmosphere, looks good, story's pretty typical but hey, if it's not broken why fix it. You just need to balance it a bit.