We travel to Valve HQ to go hands-on with the dev's innovative new gaming experience set in the Half-Life universe - it'll bend your mind

If it had a monorail and a log flume, Valve's Seattle headquarters would be the perfect theme park. The first thing you see as you swing past the corporate-looking office doors is a giant valve and the developer's reception is more like a trophy room than a waiting area. Awards and trophies fill almost every bit of wall space, and a golden crowbar stands alongside a beaming receptionist called Katie, who is so friendly that I'm sure she'd even kick me out with a smile.

While I could easily spend my morning prancing through the halls, unravelling Valve's secrets and raiding its immense chocolate stash, I'm quickly rushed off to a darkened room where waiting for me is Kim Swift, a young woman who - as well as being very pretty - is also one of the main brains behind Valve's new mind-bending puzzle-shooter-thingy, Portal.

"A lot of what makes Portal better than Narbacular Drop," she says, referencing her original college project on which Portal is based, "is adopting some of the Valve design processes. Number one, play testing; when we first got here we actually started our first play test maybe two weeks in. We'd get a good idea of what mistakes we'd made, better ways to fix the game, whether or not we're teaching players the right things and it's been really helpful. It's something that I wish I would've known in school."

The new Source venture takes Swift's college project and fleshes out her team's portal technology in a game set in the Half-Life universe. It all works by using a handy new addition to your arsenal, the Portal Gun, to fire both 'entrance' and 'exit' portals onto any flat services, which both yourself and objects are able to pass through to solve puzzles - or just infinitely fall through the floor. For laughs.

Moving over to the rigged-up PC in the corner of the room, I soon notice the results of Portal's extensive play-testing. It's easy to see that Valve's trademark, watertight balancing is also present in Portal, with early stages of the game carefully tweaked to introduce players to the concept - something that we know from previous movie trailers could well leave us on the floor with our heads in a spin.

Gently introducing me to the Portal's world-twisting gameplay, I'm first instructed by the game's ever-present, disembodied female voice to move blocks through already-present portals, something that gets me used to the idea of zipping through walls and falling out of ceilings. The voice-over itself is fantastically acted and provides some amusing lines, such as an absurdly-long acronym for the simple switches I'm dropping blocks on.

After taking a short elevator ride - something that helps separate the game's different 'puzzles' - things get a bit more advanced. In this particular room, the two 'exit' and 'entrance' portals flick between locations, which after a brief moment of head-scratching I time to snatch a block from a sealed-off room and put it on a switch on the other side of the room. While puzzles like this could very easily become confusing, it was thankfully clear from my hands-on time that Valve has taken a lot of care presenting tasks in an obvious manner, while putting up an adequate level of challenge at the same time.

And puzzles get a lot trickier as the game progresses. Unlike the dark, fleshy corridors in Prey, Portal's clean, white environments have been presented with a simple, easy-on-the-eyes attitude, providing Valve with a little more freedom to concoct wild and head-spinning tasks. Once I'd got my hands on the all-important portal gun, extra set-pieces and apparatus were added to the mix, such as moving platforms, bouncing energy balls and rotating sentry guns. But what impressed me the most was the subtle efforts Valve has made to keep everything obvious - visual cues like shadows and lights on the ceiling, so you know exactly where to place your space-bending hole.

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