Left 4 Dead

Game Modes

Campaign

Each scenario, or "movie," is divided into five sub-scenarios, showcasing a desperate push from one safe house to another, fighting hordes of ferocious Infected. In the first sub-scenario, for example, the survivors must make their way to the streets below and into the subway system where there is a safehouse, where they may recuperate and re-arm before making their way to the next safehouse. Each campaign ends with what's called a "finale", where the players must hold out against hundreds of charging Infected while awaiting rescue. Players have unlimited time to gather up supplies and figure out a plan before starting the finale, which is done by contacting the rescue vehicle through a radio.

There are four difficulties to choose from: Easy, Normal, Advanced and Expert. The higher difficulties make Infected do more damage, give Special Infected more health, and make the AI Director spawn more Infected and fewer powerups. Past Easy, friendly fire comes into effect and become more dangerous until Expert where it can down a teammate in one hit. A combination of severe accident penalties and Infected hordes dealing 10 damage per hit guarantees that only the most experienced of players survive on Expert.

Multiple visual in-game hints, including license plates, park signs, and markings on airplane equipment, imply that these locations are in Pennsylvania.  The following is a list and description of the 4 campaigns:

• No Mercy - The survivors must make their way from the rooftop of an apartment building, through the subway system, down into the sewers, and then fight their way through a hospital in order to reach the helicopter pad on top of the hospital's roof.
• Death Toll - An urban woodland map where the survivors must find a cabin and wait for evacuation by boat.
• Dead Air - The survivors make their way through a construction site before going through an airport with the hope of being rescued by plane.
• Blood Harvest - The survivors head to what they think is a military outpost only to find out it is abandoned. They must wait until the military comes back to rescue them.

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 latter of which will spawn only a few times during a game.

The Special Infected players re-spawn after they have been killed in combat (the time it takes varies depending on how well the survivors are doing and how many player infected there are), 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 and they must be out of sight of 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 based on what classes are available in the current game. The Hunter is the most common Infected in Versus, and some teams can have up to 3 Hunters in play at once.

Survival

Survivor is a new game mode released by Valve. The DLC is free of charge and comes with the two campaigns that were missing in Versus mode. Survival mode is played in one of sixteen levels available, fifteen of which are taken from the current campaigns, and includes one new level called The Last Stand, which features a lighthouse. No enemies will appear before the players start the event and they have as much time as they want to set up, mostly consisting of placing gas cans and other explosives around the map. They need to work together to survive against waves of zombie attacks by making organized runs for ammo and fighting off the horde as well as Special Infected for as long as possible; it is said that rounds will rarely last up to 9 minutes because of continual increase in difficulty as the round goes on. The longer the survivors are alive the more frequently they will be attacked by the special infected. The goal of Survival mode is to survive as long as possible and you get a bronze, silver or gold medal at the 4, 7 and 10 minutes mark.

Gameplay

Down But Not Out

To force cooperative behavior in the party, Left 4 Dead is filled with situations that incapacitate players, such as attacks by Hunter or Smoker type zombies, or pitfalls such as falling off of ledges, where another character must come and help the victim back to their feet. 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, are left too long, or if they have already been disabled twice before, they will die and become a spectator. Some instance, such as being hit by a Witch, instantly knock players to the ground. If the players reach a safe house or find one of the many "respawn rooms"  they can let dead players come back into the game. Players do have the option to play solely with bots, though the game can support up to three other human players in online co-op for the campaigns. 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, even behind you or from above. In addition to the standard zombies, advanced mutations occasionally appear, each causing their own type of havoc. The "Boomer" type of Infected 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 shooting or meleeing the zombie off of them (shooting a survivor being attacked by a Hunter or Smoker does no damage to them). The "Smoker" type has the ability to release smoke from his body when killed, blocking voice communication and view distance, but 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 essentially a large hunk of muscle capable of destroying an entire team within a minute.

The health for all of the characters 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.

A.I. Director

Not dead yet? More zombies!

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. The experienced group got more 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 diminished on the higher difficulties, obviously due to there being more zombies per level.

Not Completely Random

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.

Narrative

Instead of choosing a storyline for the players to follow, the developers decided upon using a looser form of narrative similar to the style of Portal.  The opening cut-scene serves to be as much a training video as it is a introduction to the world.  The four playable character's personalities are established and four of the five special infected are shown, excluding the Boomer.  Additionally concepts such as the melee knock-back and the flashlight are shown.  The only information that the player is given is that the infection has spread quickly, taking only two weeks to turn the majority of civilization into zombies.

The campaigns themselves do not have any voice over or story.  In the opening seconds of the first section of the campaign, one of the player characters will give an overview of the objective for the campaign after a brief fly-by.  Otherwise all of the story occurs via scribblings on the walls of safe houses by previous survivors (see picture on left).  These can be humorous messages such as a forum style flame-war with the conclusion of "I miss the internet," or can help to give the players motivation on what they do want to achieve. A few of the scribblings are hints at achievements, such as killing 53,595 infected.  The source, cause, and type of the infection is never directly discussed in the game, but there are hints that it was caused by one of the mega-corporations now trying to clean up the mess.

Special 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 Special 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 - Obese and grotesque, the Boomer is filled with infected vomit which will alert all the nearby Horde when covered over a survivor. Being blown up in range of the survivors can also alert the horde. The Boomer can be considered a liability to the Infected due to the fact that is has the lowest health of them all. Boomers can also be knocked back a fair distance by hitting them with your weapon.
  
  

  The Boomer.
• The Hunter - The Hunter is a fast and nimble Infected, capable of climbing up walls to reach high points in maps so that he can then pounce survivors and rip them to shreds. The Hunter has a small amount of health, but he makes up for this with high speeds and quick reflexes.
  
• The Smoker - Equipped with a uniquely long tounge, the Smoker can wrap it around survivors and pull them towards him. Along the way, two things can happen. If the Survivor that's being dragged hits an object, they become constricted and the Smoker begins choking them. If they victim manages to be dragged all the way to the Smoker, they will then get mauled over and over again by the Smoker's claws until they are rescued or killed. A smoker also releases a cloud of smoke when killed, hence the name.
  
• The Tank - By far the most deadly Infected, this behemoth pounds survivors with its huge fists as well as tossing parts of the environment at them. He has a strong weakness to fire and fire-related weapons. He is the only infected that can't be killed with one headshot. The Tank likes to make his presence known by constantly yelling, and battle music plays whenever a Tank has entered the game.
  
• The Witch - The only female Infected, but rare and extremely dangerous. Although the Witch may appear harmless, she can kill a survivor in seconds if disturbed by a flashlight or close contact. Witches are easy to spot because of their moaning and crying, and they can often be placed in obvious spots such as doorways or even the saferoom.
  

Valve has stated that there will be future updates, including new maps and new Boss Infected. Each is uniquely grotesque and possesses unique abilities. There is also a female infected known as “The Witch” but is only AI controlled in Versus mode and is seen more on the harder difficulties. The goal of the player controlled zombies in Versus mode is to stay alive as long as possible and collaborate with the hordes of regular AI infected and other special infected to stop the four survivors from reaching the safe house. If killed, the special infected have to wait for the respawn timer to run down.

Weapons