Your locked in... you hear some wierd nocking... you go to investigate, then you run screaming back to the spawn-room and build a crazy tall sandbag wall!
It was a normal day in the testing facility until something went horribly wrong!
Thats just a dramatic way of saying "Dude, theres some zombies, a wall, and some guns. Have fun."
You should avoid the Tank... (your ex-boss)
Feel free to criticize!