The exact progression through each level is not known from the start.
No place is safe for too long in this game, however, and once the zombies start coming, they keep coming, no matter where the players run or hide - until the heroes make it to the next safe room, to end the level. In a standard game, players begin each level in a safe room area, where they can stock up on supplies and agree on a general strategy before moving out into the zombie-flooded city streets. Computer-controlled companions are available for solo players, but online multiplayer co-op games are a main thrust of play. They'll need to trust one another and work together, as it takes teamwork to survive in streets overrun with rampaging zombies. The characters are identical in the way they play, although outwardly they appear largely dissimilar and unlikely to get along well at first. Living up to the multi-faceted meaning of the title, Left 4 Dead features four playable human characters, who have been left behind to survive in a city with a rapidly growing population of people-eating undead.
