I love a good zombie game. The objective of the game is to survive until the last level. The best way to survive is to maintain your barrier hp high while getting more weapons and people to help you survive. Each wave of zombies gets more difficult so be ready for them.
A tip would be to try and get as many survivors as you can early on so that you would be able to do work faster. Your barricade will be repaired more efficiently when you have more people. Try to get the best guns possible. The uzi and shot gun are a nice combo.
Move Up,Down,Left,Right: W, A, S, D keys
Shoot: Left Click
Reload: R (or shoot your last bullet for automatic reload)
This game may not be suitable for younger people as it contains gore and violence. You have been warned.