Left 4 Dead

Gameplay

Campaign

Each scenario, or "movie," is divided into five sub-scenarios, showcases a desperate push from one safe house to another, desperately fighting hordes of ferocious infected. During each movie, there will be a table (spawned by the AI director) that houses an M16A3 assault rifle, an M1014 automatic shotgun or a "hunting rifle" (M14 rifle, semi-automatic only, scope). There is also a molotov cocktail and pipebomb, the latter of which, when thrown, flashes and makes loud noises to attract the infected towards the bomb. In this sub-scenario, the survivors must make their way to the streets below, into the subway system where there is a safehouse, where they may recuperate and re-arm.

Versus

Aside from the normal campaign mode where only the survivors are playable characters, there is also a Versus mode. In this mode human controlled players can play as advanced zombie mutations, known as Infected, which come in four types: “Boomers”, “Hunters”, “Smokers”, or “Tanks”; the last of those will spawn only a few times throughout the game due to his superior strength. Each is uniquely grotesque and possesses super strength and other special abilities. There will also be a female infected “Witch” but will only be AI controlled and will only be seen on the harder difficulty levels (in the pre-release demo, however, the witch appears in all difficulties). The goal of the player controlled zombies will be to stay alive as long as possible and collaborate with the hordes of regular AI infected in order to stop the four survivors from completing the level. If killed, the infected bosses have to wait until the AI Director spawns them in game, as opposed to the survivors who can “bleed out” and fight off enemies from the ground using  a handgun.

The boss zombie players re-spawn 20 seconds after they have been killed in combat, being able to choose their re-spawn location by walking around as a ghost until they find the perfect ambush spot, though it must be a certain distance from the survivors. A player cannot pick which class he will play in Versus mode and it will be chosen at random by the AI Director.

While only No Mercy and Blood Harvest are officially playable in versus mode, it is possibly to play Death Toll and Dead Air in versus mode in the PC version by copying and renaming a number of files.

General

Teammate survivors can revive a fellow player up to three times before that player dies for good and goes into spectator mode until the players reach a save house or finds one of the many "respawn rooms" which when open let dead players come back into the movie.  Players do have the option to play solely with bots, though the game can support up to seven other human players online. If players come in or drop out of the game, the developers claim the AI is solid enough that the switch is almost unnoticeable.

Players must work together to fight off zombie attacks, which may come from any direction due to the nature of the AI Director. In addition to the standard zombies, the advanced mutations occasionally appear, each causing their own type of havoc. The "Boomer" type zombies will vomit on players, which causes the player's vision to become a green haze while attracting other zombies. The "Hunter" type will pin down a player, making it necessary for the other teammates to come to their rescue by blowing the zombie off of them. The "Smoker" type has the ability to release smoke from his body while it also has an extremely long prehensile tongue, which it uses to grab players around the neck and strangle them until they are rescued or dead. The "Tank" type is simply a large, fast, extremely powerful zombie.

Because the game is intended as cooperative, there are a number of situations that require teamwork to survive, such as attacks by Hunter or Smoker type zombies, or pitfalls such as falling off of ledges, where another character must come and pull them up before they lose their grip. A player losing all health is not instantaneously fatal, as they will fall to the ground able to fire a sidearm while they wait for a teammate to assist them. If they are hurt too much during this time or are left too long, or if they have already been disabled twice before, they will die. All characters' health is displayed in the HUD, and characters automatically speak when doing certain actions to facilitate teamwork. Messages are also displayed for players in need of assistance, such as those pinned or caught by advanced zombies. Players may use items such as medkits on themselves or on other teammates.

While the AI Director creates a dynamic experience, avoiding scripted enemy attacks that can make a map feel stale and boring after a number of playthroughs, as well as adding bonus items such as weapons randomly in a level, there are still some slightly scripted events. For example, in the first subsection of the Mercy Hospital scenario, there are a number of abandoned cars outside the subway entrance that if disturbed by the players by either bumping into them or firing at them will trigger the car alarms, causing the AI Director to spawn a zombie attack which the players must then fight off or escape. Valve has also mentioned "crescendo" events that occur during the game, where the four survivors have to survive an onslaught of infected for a specific amount of time. These crescendo events are one of the few scripted events in the game.

Director AI

An important concept in Left 4 Dead mentioned briefly earlier is it's "Director" AI. The Director controls the pacing of the game to match the players style and skill level. It also adds variety to the same levels since the director will spawn infected in different places. At E3 2008 Valve showed the first level played by two different groups of different skill levels. The inexperienced group stumbled through the game and director sent them less baddies. 

You're playing better? More zombies!

The experienced group got more and tougher bad guys, and led to more on-screen explosions. Specifically a docile gas station for the first group became an exploding war zone for the better group. One Turtle Rock developer stated that they want the player to feel as though they've just barely survived.

The Director uses a system of pacing and what is considered a player's "stress level" as determined by your actions to give you the best experience possible.

The game plays with peaks of high tension and valleys of quiet fear with few zombies, thanks to the Director reading how stressed the player becomes. When the Director 'feels' that you are being overwhelmed, the gameplay will ease up a bit. Players can have fun with the Director by entering sv_cheats in the console and setting the Director to constant "panic" mode. After that, the next event in-game will cause an almost neverending tide of hordes to attack the player. This proves the Director controls the game's pacing.

It is assumed that the changes in stress for the player become diminshed on the higher difficulties, obviously due to there being more zombies per level.

Boss Infected

The game has regular Infected which are encountered throughout the game and have no special abilities or traits at their disposal. Apart from them there are 5 Boss Infected in the game, 4 of which are only playable in Versus mode and all of which will be seen (non-playable) in the Campaign mode.

• The Boomer
• The Hunter
• The Smoker
• The Tank
• The Witch

Valve has stated that there will be future updates, including new maps and new Boss Infected.

Weapons