When you put Valve and Turtle Rock studios together in one place you're going to get something pretty special out of it. In this case it's like the holy amalgam of peanut butter and chocolate, a tasty treat that's sweet but also chewy. In video game terms I'm talking about the zombie survival horror first person shooter: Left4Dead.

You and a few other survivors are immune to a virus-like infection that manages to turn the rest of the population into blood and flesh-crazed zombie-like monsters. These monsters are hell-bent on snacking on the populace that remains and survivors must attempt to get from a starting location to a pre-determined end point in 4 decent sized campaigns spread out over 5 missions per campaign.

The game doesn't have a cinematic story mode; you have very little in the way of story/background to get into. This is good old fashioned zombie shooting action the way that it was meant to be done. A slight cut-scene begins the campaign and a small one ends it, everything rolls with credits (the credits being full of game stats) and that's about it in terms of story.

You have four archetypal zombie horror survivors that have no discernable in-game statistical advantage, presumably so Valve and TR could keep the game's balance fairly in check regarding the survivors.

Bill is the ex 'Nam vet who is fighting a new war against a soulless enemy.

Louis is the disgruntled office worker who is just trying to survive.

Francis is the hard-bitten biker dude with tattoos and a loud mouth.

Zoey is the hot college girl who manages to be sensible and tougher than she looks.

Left4Dead works well as a pick-up and play game, it's easy enough to learn the controls and Valve have made sure that the console version is just as good as the PC game in terms of playability and design. You can carry one extra weapon and you always have a pistol (with infinite ammo) as a sidearm. There is a shotgun, assault rifle, sniper rifle, machine pistol and a few grenade types (pipe bomb and molotovs) to choose from.

Each player is allowed one bottle of pain pills and one health pack.

Left4Dead fills non-human player spots with AI and allows you to play offline (via split-screen) or online, cooperatively or via adversarial matches with friends or strangers over Xbox Live. This is one of the true cooperative gameplay experiences that I've come across in a long time. I can't sing the praises of this kind of game or gameplay enough; being able to battle against the AI (or other human players) with up to 3 other friends is something that other developers need to take note of.

Valve have managed to mix in cooperation even in versus mode (discussed later on) and they've crafted an addictive blend of adrenaline fuelled gameplay to go with it. Whilst you can shoot the enemy with your friends, the true cooperation comes from being able to pull them up if they get knocked over the side of a building, give them pill bottles or even heal them if they're too badly injured. You're going to need to work together as well, since the AI has a habit of doing just that against you.

A Smoker will drag off a team mate so they can be pounced on by a Hunter, or vomited on by a Boomer. When a Tank appears, you will often need every gun in your rag-tag group of friends to survive, because they are true engines of destruction.

Built on the next iteration of the Source engine and using Turtle Rock's next generation AI technology, Left4Dead has some serious stuff going on under the hood that might be missed by the average player who just wants to shoot things. I actually love technology like this so I poke around and read up on the various systems, not from a programmer's point of view though - I suck at code. The biggest innovation in gameplay terms is the AI Director, this disembodied figure stalks the shadowy corridors of the game environment and alters the game depending on how well (or badly) you seem to be doing.

If the AI Director thinks you are moving too slowly or getting bored, it will pep things up a little by hammering your group with a panic event. It might decide to generate a subtle musical clue to warn you, or just leave you to scream as a horde of 28-Days later zombie sprint-runners come charging at you from various spawn points that the Director thinks are amusing.

Perhaps you need a few new guns, in that case you might find a stash of them on a table in a room if you explore. The same can be said for health packs, pill bottles and the explosives. The AI Director controls normal zombie placement as well, so every replay is different in terms of the pre-placed enemies that might be lurking around waiting for you. Panic events aren't always triggered at the same time and the AI Director also has access to the sound effects, visual effects and ambient noises.

Each campaign also features a bunch of 'siege' style events (crescendo points) where you have to trigger a particular one to progress. Raising a massive metal door to get out that alerts the horde, or something similar is the order of the day here and these events are great ways for a seasoned team to show their stuff. The last mission of each campaign culminates in a big stand off against wave after wave of infected whilst waiting for a rescue vehicle to show up.

It's not just the AI in Left4Dead that's excellent, the level design is superb and the graphics are great. The model design for the infected and the survivors is spot on. The ambient special effects showcase that the Source engine has gone from strength to strength, lighting is good, the little tricks such as greying out the screen to a kind of TV-static effect when low on health gives it an edge of desperation and a cinematic quality. The animation is excellent, each of the survivors personality shines through their character model and its actions.

The infected animation is superb, combined with the physics it just shows what you can do. These zombies aren't going to die the same way each time you shoot them, some of them keep on running for a while and slowly just come to a stop when they finally collapse just before they reach your character. Or they careen off to one side and bounce off a wall, it's all great stuff. The various weapons have different forces as well, so you can imagine the shotgun is more likely to knock the infected back than the pistol.

Sound and music is top notch, the ambient sounds are great, the music is great and the starring role goes to the voice acting for the various survivors, their dialogue and interactions take the edge off the bleakness of the horror and they have a tonne of voice dialogue that is context sensitive depending on the situation at hand.

As previously stated, L4D can be enjoyed on your own with 3 AI replacements against every campaign or with up to 3 other human friends over Xbox Live. You can even play System Link or 2-player split-screen (coop only) on the same Xbox360 with a friend.

Where L4D really shines (though only two campaigns are active for it at the moment) is when you have a round-robin style 4-v-4 versus game. 4 survivors and 4 infected players. This is why the game is such a blast. Whilst the campaign plays out just like in single player with full AI Director control over the core infected, the human players on the enemy side are in charge of the Boomer, Smoker, Hunter and Tank as they attempt to stop the survivors from escaping.

Then teams switch and the infected are now against the survivors with their roles reversed.

There's a great deal of cooperative teamplay on both sides of the fence, the infected can work together to navigate the various maps and lay ambushes for the players using the right time to strike, especially during a particularly nasty panic event or siege event. Pulling a survivor from a mounted mini-gun as a Smoker whilst the Hunter rips them to shreds is a great feeling, then to see a Tank come crashing into the game to mop up the rest gives you the satisfaction of a job well done as an infected player.

Or a Boomer that manages to vomit on a couple of survivors, rushes the rest and explodes mid-group covering everyone in bile is great especially when there are a couple of Hunters on the loose who can leap in during the green-tinted vomit-confusion and rushing zombie madness. This can really wound or nearly cripple a group of survivors and cause mass mayhem and panic in the group.

If I were to pick faults with L4D it would be that the game has a few little online niggles, most of which Valve have picked up on with hotfixes and patches recently. There were some balance issues with the infected and several exploits allowed hackers to use the console on Xbox 360 matches to screw with people's games, summoning in hordes of Tanks or so on.

The faults though:

1. The weapons - they're all very much 'same old same old' school of guns.

2. 4 campaigns even with 5 missions doesn't seem a lot.

3. Only 2 campaigns in versus mode at the moment.

4. 4 main characters are not different in terms of statistics, only a very minor niggle

5. A few lag issues in multiplayer, mostly on local host servers.

6. Need more levels where it's possible to snipe from, since the hunting rifle seems redundant mostly.

You really can't even call them faults. All in all Valve and Turtle Rock have made the kind of game that is going to stay on my 360 HDD for ages and I know with Valve's rep, there will be some pretty slick DLC coming from them in the near future - the PC side of things will get that for free but I highly doubt that they can offer the same content for the 360 without charging for it.

Microsoft isn't fond of that.

Left4Dead is a step in the right direction for so many shooters, the AI Director is the best thing since self-slicing ninjabread and versus mode is immense fun. Allowing you to take a break by letting the AI control your character until you come back is another nice touch, since the AI tends to be pretty damn good most of the time. This game can only go from strength to strength.

If you don't own it, you need to get it.