Zombies pour out of the fog in waves — faster and in greater numbers each round. Survive as long as you can. Clear a wave, catch your breath, and brace for the next.
Watch your ammo — you reload with R, and your reserve only tops up between waves.
Grab the glowing green medkits to restore health. They drop between waves and sometimes from the dead.
Switch weapons with 1 Rifle, 2 SMG, 3 Shotgun (or the mouse wheel). The shotgun shreds up close; the SMG chews through crowds; the rifle is the all-rounder.
Aim for the head — headshots deal massive damage and can drop a zombie in one shot.
Use crates, cars and containers for cover — and keep turning. They don't only come from the front.