You need add these 3 , if You want make your game better.
1. Zombies are appearing from anywhere ( random position ) , make a slot, where zombies will appear. That will be better.
2. When you're shooting, gun sound is very silent, make it louder, or make options bar where players can change game or guns sound, by their self.
3. Add some blood. Zombies are just dying and that's it ? Where's blood ? Make some blood. I like blood.
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And you can add these, if You want.
1. Some new zombies, for example: When player reaches score 500 there will appear faster zombies, or something else.
2. Can add more guns.
3. Add reload function to gun.
4. Music pretty annoying.
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You can also add some medals, and high scores on newgrounds system. So players will see other scores. Will be great if You gonna add medals.
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And one bug. Some zombies are appearing in that point, where I'm standing. So it's kinda stupid. You can fix it somehow if You want to.
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That's what I'm thinking about this game. Peace.