You can retry the level and this will clear some of the zombies that don't go away but the game will still be hard. On the next retry your gun won't work so is impossible to beat.
Strange Glitch: 'Normal Guy and Zombie Barracade'
Start a round, right click and select 'play' when zombies are in the screen, the game will reload but the zombies will still be on the screen attacking the barricade that isn't there. Start the game again, the zombies on the screen will zoom around a bit and end up standing against your barricade walking on the spot facing where the new zombies will come from. Unfortunately it doesn't help it seems to make the game harder if anything obscuring your vision and you can't finish any rounds due to the fact that you can't kill these zombies but they're still there. If you do this glitch on round 5 (not sure if on other rounds) a ordinary looking guy in with pale yellowish skin and brown hair with green eyes will appear and attack you (hoodie and green trousers?) you can kill him.
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