Game development is very hard work.
You need to know hard languages, like assembly, C, and C++. There's rigorous math. Any inefficient algorithm needs to be found and stomped. There's long hours, maddening bugs, delays, burnout.
So the developers play. Valve software of Washington State is a very progressive company - you don't report to a manager, they recruit in-house staff who can switch projects of their own volition at any time. They're open to having their games scriptable, extensible, and easy to modify. So the teams, in addition to taking required breaks to recharge, were also messing around with their Counter-strike game, playing around with different variants. The one they had the most fun with was one where you just simply abandoned all military strategy and sniping and went around mowing down hordes of crowds whose only weapons were fists.
Because mayhem is fun. There was a certain joy in being able to just run around with a gun taking out crowds, without worrying about being sniped or blown up by some camper named BlAz3-it420 cackling maniacally through his headset into the earphones of yours.
Turning the idea over in their minds, and remembering the "runner" zombie genre of 28 Days Later, they realized if they made a zombie variant that attacked, charged and pummeled a living human to death, they had a plausible story and reason to try to survive such an area. A tiger team got together and asked for permission to work on this and got it - within a week they had a pretty playable level of a demo.
Thus began Left 4 Dead.
The intro movie to the first game explains it all beautifully - four survivors are caught in a city, armed with pistols, shotguns and an AK-47. As they creep through eerily abandoned streets, they encounter first what they think is a sobbing little girl, but it turns out to be a Witch (vide infra) and Bill, the old Green Beret of the group, orders Zoey, the lone female and ex-news team tech, to switch her light OFF. As they back away carefully so as not to startle or arouse the very dangerous zombie, the computer systems analyst Louis and ex-biker gang member Francis stand guard, unawares, outside. A horde starts chasing towards them and Louis pops his head in to warn them. The light on his shotgun startles the Witch, and they run for it, watching in horror as she starts to tear through the steel door. They fight through the initial horde, but in the melee they encouter a Smoker (vide infra) who strangles Bill, and then they knock a leaping Hunter (vide infra) into an alarmed car, whose alarm raises a giant horde that comes running from all directions. To further add to their problems a Tank (vide infra) smashes cars and zombies out of the way, and they have no idea whether to run or to fight. They choose both, firing hundreds of rounds at the mass of muscle and violence, eventually deciding that they can make it up a fire escape but the Tank would be too heavy to follow. They almost all make it - the Tank leaps at the fire escape and Zoey looks to be about to fall to her death. Francis catches her hand and pulls her to safety as the Tank falls to its second death. Louis exclaims happily that they made it, and Bill ruefully points out, from what is now clear to be the rooftop beginning to No Mercy - all they've managed to do yet is cross the street.
It's one of the best game intros ever. It establishes all four characters, all key threats, shows the use of a pipe bomb to distract and destroy an entire group at once, and sets the mood which is claustrophobic, tense, and horrific.
One can play the episodes of the game in any order, but Left 4 Dead really "starts" in "No Mercy", in which the crew must fight through the city, then the subway system, then the sewer system, to get to the hospital roof and try and reach the last helicopter evacuations of survivors out of the city. (A deleted scene has the pilot say on his way out that he doesn't feel very well....)
In the DLC campaign "Crash Course", the helicopter is seen to have crashed, and they must make their way away on foot from the crash site towards the town of Riverside where they discover a hand-made armored vehicle on a lift. Of course, activating the lift requires turning on a generator.... which attracts attention...
It then continues into "Death Toll", where the four must fight their way through a highway tunnel, to a drainage systen, then an abandoned church, through the town of Riverside itself, and then finally to the riverbank itself, where they are rescued by a fishing boat.
In "Dead Air", they start out in a greenhouse, and, seeing a plane flying overhead, decide to fight their way through various office buildings and such and a ruined parking garage into the airport itself, where they fight through terminal debris and wend their way through luggage conveyors and the like to get to the gate, which they exit through to find a plane in need of fuel. In exchange for turning on the pump and holding off the zombie hordes, they'll get passage out by air.
Left 4 Dead on disc ends with Blood Harvest, where they fight through fields and forests, and end up at a deserted farmhouse, where the military arrives to collect them and take them away in an armored vehicle.
Another DLC campaign, "The Sacrifice", deals with the story after the comic book - in which it's explained that the military aren't in charge, they're barely surviving themselves. And not only are the four survivors immune to the "Green Flu" which is affecting everyone, worldwide, they're also carriers, which explains why every rescue they've had has ended in disaster. The four survivors, having escaped the military, decide to make their way to find a boat somewhere, a sailboat in which they can live and find some kind of habitable island. They make their way through seaside warehouses, and at one point have to make their way through a boxcar the military decided to capture a Tank in. The finale involves starting three generators in order to lower a bridge to let them pass on to freedom, but as the bridge starts to raise the generator stops. One character (supposed to be Bill, but can be completed by any character) jumps from the bridge and restarts the generator, saving the other three but dying in a crowd of Tanks and zombies.
Playing these episodes is pretty clear. There's a "safe house" Point A, where you're safe behind a steel door. Unfortunately, though there's first aid and weapons and ammo, you can't stay there forever. You must continue through the door to point B. The moment you leave you follow a path with some dead ends, and run into zombies enroute. To avoid the usual "cheap" ways of staying alive, there are special zombies with special powers, to prevent people from "camping" in certain points and/or allowing the artificial intelligence running the game to up the challenge level. The game ends (returning you to the beginning of your current level if you aren't on the last part, ending the game immediately if you are) if all four characters are dead, or incap'd, or some combination of the two, or completing the game successfully if any character makes it to the safe room or escape vector alive.
You have the use of one of two weapons, which you can switch out. All characters have a pistol with limitless ammo. Finding another pistol lets you "double fist" with pistol shots. You can also switch to using any of a number of long guns you might find, but only have one of at a time. A shotgun, whether a standard, chrome, auto- or military shotgun, does devastating damage at close range but is ineffective at longer ranges. The machine gun, whether it's a silenced one, heavy one, or an AK-47, shoots rapidly but does less damage per hit. You can heal damage temporarily by taking pain pills, or heal damage permanently by applying a first aid kit, both of which require you to stop and take an action. You can also throw one of two projectiles, either a Molotov cocktail or a pipe bomb equipped with an annoying beeper which causes all zombies present to chase it furiously. Players can take damage from being in proximity to a blast or caught in fire generated from either a Molotov or a shot gas can.
Players have five modes: Healthy (in which the life bar is green) with 100 "hit points". Damage to below 40 or so means the player is "injured" and starts limping, slowing down considerably. With life reduced to zero, the player is "incapacitated" or "incap"'d and lies there, able only to shoot, not use health packs or move. The player is practically helpless and unless "revived" by another player (who must come over, stop fighting, and pick the character up) has 200 rapidly draining "incap life" points bleeding out which, when it gets to zero, makes the character dead. Note that being hit, attacked, shredded by claws etc. while incap'd will make the player "bleed out" faster. The last state is a weird one. When revived, the player gets about 20 health points back and is critically injured, but is able to take actions, like heal and move. But you can only be incap'd twice. After the second time, when you stand, if your life is reduced to zero you die immediately, and this status is evident to the player by having his surroundings turn black and white with the sound of a heartbeat constantly in his or her ears.
To avoid having the players simply randomly shoot everywhere, there's the Boomer. He's basically a large, fat zombie who looks like a waterbed with legs in terms of how the insides of his body move. His primary weapon is to vomit on one or more survivors, obscuring their vision but also attracting any and all zombies in the area to attack anyone covered in the vomit. Simply shooting one causes it to explode, coating the shooter and anyone nearby (unless shot from a distance) in vomit, leading to the same problem.
Players are also ill-advised to shoot randomly by the Witch. A pale, sobbing woman zombie with long hair and long nails, any loud noises of shooting, any close proximity, or any light shined her way causes her to start growling. Unless you immediately back away and kill your lights, she will stand up, shrieking, and attack the player who "startled" her. In the Expert difficulty level, this instantly kills the character. It incaps a player at every other level, and she continues attacking. If other characters don't kill her soon, she will continue attacking until the player is dead. She can be killed by a shotgun blast to the forehead (which gives the player the Achievement "Crown'd") but that is a tricky proposition and has to be executed perfectly.
Players can be pulled off vantage "camping" points by the Smoker, a deformed looking fellow who has a long tongue which snakes out over about eighty yards and ensnares a player, making him or her helpless, and dragging him or her away from his or her position. Anyone parked somewhere happily dispatching zombies will find himself dragged OFF that position.
Another possibility the AI has is to use a Hunter. a blind fellow in a hoodie who leaps considerable distances and when he lands on a player, renders that character helpless and simply shreds that player's intestines with claws until bumped off the player, or killed.
The final "special" Infected is the Tank, a giant mass of muscle that can immediately incap a player by smashing a car or a dumpster into him or her. It can also tear up and throw rubble at players, or simply knock it considerable distances with its fists. The latter is doubly dangerous in a high building or around water, as falling off a building or landing in deep water is an insta-kill. It has five thousand hit points and is a near-unstoppable juggernaut, and truly terrifying in close quarters.
So much of this game is so wonderfully thought through. Attention to detail has been paid in lighting (which guides you through the level subtly, and adds mood), level design (claustrophobia, fear of dark places with jump scares) and mood. Smoke artistry and particle effects go beyond mapping smoke and fire and are used to "fade" light over distance in a cinematic way. Each special infected has a leitmotif or theme sound, be it the six note cello riff signalling a Boomer, or a set of three flammed piano notes signalling a Hunter. Hordes have a distinctive howl, you can hear Smokers mewl, Boomers grumble and rage, and Tanks growl from far off enough that if you pay attention you're not truly surprised. The theme itself is on a versatile scale that gets reused in multiple ways, from a berimbau and Bauhaus-style guitar somber-mood intro, to a rock-and-techno riff.
Graffiti found around the levels tell a story. Zombies fear "Chicago Ted". "We are the real monsters". Instructions from loved ones "Mike: Evac center over-run, headed to mom's farm, meet you there. Love Lucy and Kirsten." "We found mom, she tore Dad apart when he tried to talk to her. She's one of them now." A series of skulls and hatch marks show that a small group is having fun openly hunting them and tallying up the kills. Warnings, goodbyes, Bible verses. In one very amusing twist, taking a long, blind alley ends in a dead end with "Chet was here".
Initial playtesters thought the pacing of the game was too linear: that there was an iniitial "escape from point A healthy" to "climactic event at the end of the level" with nothing in between. So the designers put in features to make mini-crescendos, some of which are avoidable, and some which aren't. Shoot or hit a car with a blinking alarm light and it will summon every zombie around. But in some instances, you're stopped and trapped and must either activate some machinery or detonate some kind of bomb to continue moving, which acts as a "dinner bell" for the hordes. In the ending to one part of "Death Toll" a survivor who's locked himself in the safe room you need to get into was previously bitten and is going crazy as he's turning, starting the church bell in the hopes that the other zombies will kill off the perceived threat of you four.
Other new technologies made this game ever so much the cooler: there's a "director", a software intelligence that not only generates hordes and weapons in response to the script or any relevant activity (setting off a car alarm), it also measures the "stress level" of a given character, namely how much damage he's taking and/or how hurt he is, and how much pressure he's under, and increases it if needed. If you're speedrunning really well with no damage being taken, expect to be pounced by a Hunter and/or beset upon by one or more Tanks, as a horde gets generated.
So here you are, in the dark, worried that the reloading of your shotgun takes your flashlight off the scene long enough for you to get pounced, watching your life level and hoping you aren't coming through this too well, as eerie music and echoing zombie growls remind you of the threat ahead.
Because the game is scriptable, the Valve team decided to allow anyone, yes, anyone, to make models, or levels, or entire campaigns for Left 4 Dead, so if you play on Steam there are entire games' worth of content available from the community. Anything from replacing all zombies with Barney the dinosaur, to being able to watch Zoey naked. One campaign was so good it ascended into canon, and "Cold Stream" is an official release, also making its way to the XBox 360.
They've even opened the universe to the point where they don't care if you make derivative media. An Italian group of volunteers are putting finishing touches on "Left 4 Dead: the Movie" as we speak, and there are remixes of the theme and derivative artworks, none of which have resulted in Valve lawsuits. They built a world and people are happily creating in that world, just as how Dungeons and Dragons became an entire ecosystem of its own.
And all this from a group of guys near Seattle thinking how much fun it would be to live in a post-apocalyptic world.
Followed by Left 4 Dead 2.