More and more zombies roam the countryside every minute. Defend your compound, venture out to find a radio, then survive until help arrives.
Optionally, rescue survivors from nearby structures and arm them at your barracks, and/or research a cure for the infection.
Unlike most zombie maps, this one doesn't spawn progressively faster or stronger zombies. There's exactly one type of zombie - it's slow but one scratch kills. Like a proper zombie infestation, the difficulty comes not from stronger zombies, but from their increasing numbers.