I want to take a closer look at last year's co-op zombie apocalypse shooter Left 4 Dead. Valve’s attention to detail and high levels of polish are well known, but after weeks of entertainment, I was still trying to tease apart on a fundamental level ‘How’d they do that?’ How did Valve take two tastes that taste great – survival horror and co-op – and make them taste great together? Here’s what I came up with.

At their core, most action and action/adventure games are about fight or flight. This is in turn about spacing; more specifically the distance between you and your enemies and the distance between your current position and the next safe zone. As you move from checkpoint to checkpoint, you’re either trying to put as much space as possible between you and your opponents or you’re trying to fill that space with as much lethal force as possible.

From this basic action/adventure premise, developers of survival horror games tweak the formula in a variety of ways. One is how quickly the game lets you move (the vast majority of survival horror games slow your movement speed relative to other titles) and the other is whether you can both shoot and move at the same time. 

Your movement speed determines how much space you can put between yourself and your enemies; the ability to shoot and/or move determines whether you have to choose between fight or flight (Resident Evil) or whether you can do both simultaneously (Dead Space).

Another is managing an inventory of scarce resources – health, ammunition, explosives, etc – that you need to stay alive as you make your way from point A to point B.

How does Valve handle these elements?

With regard to movement, Left 4 Dead lets you move fairly quickly compared to more traditional survival horror games. But that’s not enough to keep you safe, or even to maintain the illusion of safety. 

The basic infected move even faster than you do, and they periodically swarm you in large numbers, making it difficult for you move. You can’t clear an area of enemies and then relax indefinitely, because the game’s ‘AI director’ will keep pushing you forward by throwing more and more Infected at you. And while the initial supply of ammo for your primary weapon will seem rather generous compared to traditional survival horror games (it’s even unlimited for pistols), the sheer number of Infected in Left 4 Dead functions almost like a countdown clock, tension steadily mounting as your ammo ticks down.

As long as you can maintain some space between yourself and the infected, you’ll be safe. But by pushing survival horror in a co-op direction, Valve forces you to manage a third space: the distance between you and your allies. The game’s user interface, which is far more elaborate than Valve’s previous titles, foregrounds the condition of your buddies at all times; because your group is only as strong as its weakest link, you have to monitor your allies’ status and health as well as your own.

As for the survival horror element of scarcity, here it means that each of you can carry just one health pack and one bottle of pills at a time, there’s no use in hoarding them for yourself, because the collective health of your entire unit is paramount.

And it’s not just the Survivors whose gameplay is built around movement and spacing. The five types of Boss Infected don’t have guns, but four of them nevertheless have ranged attacks, and in what might be Valve’s shrewdest touch, four of the five Boss Infected can immobilise you in a way that requires the assistance of your allies to get you back on your feet.

Playing Left 4 Dead opened my eyes to how little information other team-based shooters provide about the location and condition of our teammates; if they’re not in our cone of vision, the only info we have is what our fellow players tell us over voice chat.

By carefully providing more information through the game’s UI – right down to seeing our buddies’ glowing outlines through solid walls – Left 4 Dead explicitly reinforces the opportunities for the kind of teamwork that embodies the all for one, one for all esprit de corps necessary to make it through a zombie onslaught without ever feeling it’s enforcing that teamwork.

It’s a subtle difference, but it’s also an important one, and within the context of survival horror, it feels like a crafty new direction for the genre. As the coach of my beloved Los Angeles Lakers likes to say, the strength of the pack is the wolf and the strength of the wolf is the pack, and given the manner in which Valve has designed Left 4 Dead, joining the pack has never felt quite so rewarding.