It might get a bit repetive after some time, but heck, it's a kind of a zombie shooter. But you should use another background from time to time . It gets boring soon to always shoot the same zombies on the same field with the same music in the background.
This game also misses some upgrades. You know, like upgrading the barricade, ammunation, or equipping survivors with weapons you don't use. Upgradeable stuff makes this kind of games really interesting. Also it misses a display for the ammo left before you got to reload.
On the plus side, the graphics were good, and the system of Headshot / ordinary hit is pretty cool. Probably change the Timethingie, it looks a bit silly if the game tells you that you got 700 Time. Another plus is that your survivors got different skills. They're not just blank faces, each one got his own set of skills.
Well ... to the point, I would recommend you add a display for the ammo left, probably find one or two other backgrounds and add them as well as you should definitely add some upgrades. Some suggestions would be:
- Ammo Upgrade for Guns (For example, +1 bullet for the Remington, +5 for automatic guns, none for the grenade launcher)
- Incindary ammo for automatic Guns (AK-47, if I remember right, and its friends)
- Increased damage for guns
Well, those are just examples. And too be fair, as soon as you get the grenade launcher, you almost won the game. It's pretty awesome. Another idea would be to set different reload times for different weapons, which you could balance with certain upgrades.
For this game, I would recommend to give the character some mechanics skills instead of some kind of shops. Well, after all this are just some suggestions. Hope this will help you a bit with future games.