Team Fortress 2

Another lesson was letting the gameplay inform the design. The stylised art - TF2's most striking facet - was a direction signposted by the game itself. "We always had a nagging feeling we weren't doing art direction right in all of our titles, even with Half-Life 2, where we were quite happy with the art direction," says Walker. The problem was conceptualising artwork before the team really knew what the product was - sort of like colouring in before anyone's drawn the lines. Done the other way around, Valve's able to design things specifically to solve game design problems. Character classes all have distinctive silhouettes so you can spot them at distance, while player models are coloured in a gradient that's at it's brightest at chest height - because human eyes focus on areas of high contrast, and that's where you need to look to see what your enemy's carrying.

TF2's classes are instantly recognisable, and there's no obvious dead weight.

The art's working hard everywhere you look - the Heavy Weapons Guy obviously has more health than the Scout because he's that much bigger. And of course you'll realise what the Medic's medi-gun does - red crosses flow along the stream it pumps into your team-mates. "100% of playtesters knew what it was the second they saw it," says Walker. Tellingly, there was only one point in our entire playtest that we had trouble identifying enemies, and by the time we noticed, Valve was already halfway through apologising for the level's unfinished lighting conditions. The benefits extend beyond characters. Team Fortress' most iconic map, 2Fort, is a pair of opposing bases separated by a watery pit with a covered walkway, but now they're farms, concealing underground spy-bases. As a result, you can easily gauge how much danger you're in. Within your own spy-base? Then you're comfortable. In the farm? Bit of danger. In their farm? Fair bit of danger. In their base? Oh, you're dead.

Then there are the classes themselves. "The whole point is that each class is a bottled-up experience, and that that experience is unique within the classes," says Walker, who's concerned that Team Fortress Classic - the Half-Life 1 "stopgap" mod released just after Valve acquired Team Fortress Software in the late '90s - didn't do a good enough job of this. TF2 aims to play into the variety of skill-sets that exist among players, not least because Valve knows that in multiplayer the content you're consuming is generated by other players, and it's more compelling if it's diverse in nature. So there's the Soldier, a fun entry class for rocket-jumping Quakers and a powerful attacker, while the Pyro's almost a melee class, with its flamethrower doing more damage in close quarters, and the Demo Man's a versatile choice for players who want to move between attack and defence, thanks to its grenade launcher's mixture of detonation options: on impact, after a few seconds, or upon remote input. The Sniper, bound to be popular, gets more punch out of its high-velocity rounds if you stay zoomed in for a few seconds at the cost of peripheral vision. Tempting, but dangerous.

The Heavy Weapons Guy, currently everyone's favourite (we resisted the urge to do "Who touched Sasha?" jokes [too late - Ed]), is riotous; slow-moving, and slow to spin up its chaingun, but with the option of moving even slower with the barrel always spinning. More interestingly, it also plays into another area Valve's keen to improve on: relationships between players. Couple a Heavy with a Medic, who can constantly rebuild his health while gaining charge towards a temporary burst of invulnerability, and you have a compelling two-man unit within a larger team. "The thing we weren't expecting is there's a lot of skill to being a good Medic target [in this case, the Heavy]," says Walker; "being someone who can keep an eye on your Medic and try not to take corners too sharp to maintain the medi-beam's line of sight, and things like that." It's become "probably the second or third most popular class" at Valve, he says. We quite liked the Engineer, as it goes - setting up sentry guns and ammo dispensers, trying to plant teleport exits in critical areas of the enemy base, and whacking things with wrenches to increase build speed.

For Walker though the first port of call was, for a good while, the Spy. The Spy can disguise himself as any enemy class he likes (he appears to his team-mates as a Spy with a mask on - another example of using art rather than an icon), can make himself invisible for a short period of time, and can kill enemies with a single knife blow to the back if he can manoeuvre himself there. The problem is that he can only strike while visible and undisguised. But Walker likes him because he "rewards anyone who really understands how other people play the game". He's right - it's not as simple as picking a class and waltzing in amongst the enemy, because they can spot suspicious behaviour a mile off. "Why's that Medic running into our base?" A Heavy, on the other hand, trotting backward down a critical pathway, could just be one of yours retreating to the comfort of a common area.

Phong shading - the glow around the rim - helps you pick enemies out from the background easily.

For now though, Walker's favourite is the Scout. "I used to play Soldier a lot and Soldier's all about using rockets, because you're not terribly manoeuvrable - rocket-jumping aside - and the Scout is sort of the opposite, where it's all about how much you can keep on the move and still aim effectively," he explains. "If you can aim well moving in fully three dimensions then you can be incredibly powerful." The Scout can double-jump, allowing him a route over the covered walkway rooftop in 2Fort's neutral zone, and moves fastest of all the units. Our best moment comes when we actually jump over a Demo Man's grenade and use the Scout's Scatter Gun to put pay to him. That's another bit of learning: each of the classes now has a secondary gun you'd actually bother to use.

This passage of play brings us back to the point about relationships. Whenever you're killed in TF2 (we were lying - the second grenade got us), the game crash-zooms on your killer, telling you who it was. If he picks you off enough, he might become your nemesis. Take him out and you gain a revenge kill. The game tracks this stuff, and it sticks in your head, and feeds into how you play - in my case, with the desire to stick a sniper round between Iikka Keränen's videogame eyes. Unfortunately, I don't get to achieve this (despite lots of standing around on the battlements). Instead we switch to Well (where capturing and holding control points over time is the goal, rather than capturing items within the base and ferrying them home), and then to Hydro.

Hydro is Valve's attempt to make a map with greater replayability. There are six control points on the map. Each team starts off by owning the two outer-most ones, while the four in-between are randomly distributed. The game then picks two of these four and you fight over them, with all the other routes blocked off. "If 2Fort's loading or de_dust [Counter-Strike's most famous map] is loading, you can run through in your head what you're going to do before you even spawn," Walker points out. "You don't get that with Hydro." If you force the enemy back and take over their control point, the game switches the action to another combination, perhaps with you defending the ground you've just taken. When one team owns all four of the middle grounds, they can try and take down the final base. Rounds fought over individual combinations take five or six minutes, and the game has ways of legislating against stale-mates - switching to another pairing, with differing topography, or reducing everyone to a single life rather than the standard timed respawn.

It's all wonderfully gory, with tremendously enjoyable physics underpinning the violence.

So, through a process of iteration, the developer finds itself with a team-based, multiplayer first-person shooter with nine character classes, six distinct maps - some with differing win conditions - and a decade-old community of players waiting to apply their knowledge to the destruction of others, that's not only less intimidating than something like Counter-Strike, but strikingly accessible. At the end of each game, you get a page of stats, and the game's constantly informing you when you break your own records. "What we've found," says Walker, "is that every time we've said to ourselves, 'no, you don't just get to add a HUD element, you don't just get to put a bracket around that, you don't get to put a logo above everyone's head,' we tend to solve the problem in a more interesting way. The game's more fun for it." The best thing they've grasped, though, is that when a round ends, and you've won, it's fun to have a few seconds to jump around beating up your enemies with impunity while they wallow in their loserdom. It's something Valve hopes to do to its own rivals when TF2 launches later this year. On this evidence, they might just get the chance.