Portal is a particularly interesting game to review, as this first-person shooter-cum-puzzler is served as an aside to the main bulk of The Orange Box. It certainly isn't a game to be shunned as just another gimmick-ridden throw away though - conversely, it stands out as a worthwhile game in its own right.

Developed from concepts pioneered by the 2005 freeware title Narbacular Drop, Portal pits places you in the role of Chell, a previously undocumented character in the Half-Life universe. The game entails nineteen self-contained tests, each more taxing than the last and the concept of the portals themselves is introduced at a reassuringly steady pace, which is a relief considering their perplexing nature.

The portal gun (or Aperture Science Handheld Portal Device) is obtained early on and this device, the heart and soul of the game, enables you to 'shoot' two different portals - one blue and one orange. When these two portals are fired at any flat, suitable surface, they create a virtual 'tunnel' between them. These portals are two-way, so you can use either one as an entrance or an exit. On paper this is simple enough, but when coupled with some imaginative puzzle design it really starts to shine. This being Valve's first attempt, although not half-bad, it barely scratches the surface of the concept (as Gabe Newell confesses in the developer commentary supplied with the game).

Portal starts simply enough as you are introduced to the various elements that make up the game's puzzles. These include giant blocks of varying heights, toxic waste, gun turrets and plenty more - all of which can be traversed or manipulated with clever use of the portal gun. A robotic voice that goes by the name of GLaDOS (Genetic Lifeform and Disk Operating System, wonderfully voiced by Ellen McLain) guides the player throughout the game with a regular commentary. This isn't just some irrelevant babble though, instead it functions as both a clever plot driving device and adds some much-needed humour into this otherwise flat experience.

The puzzles themselves get really interesting when momentum comes into play. Jumping from a height through a portal on the floor, for instance, throws you out of the secondary portal at speed. This momentum effect stays for as long as you can retain it, opening up scope for some crazy combinations. Some puzzles require you to open portals mid-air and even perform some 180° spins in the process. Playing through Portal is a gaming experience like no other. It has that undeniable Half-Life feel to it, yet plays like a fully-fledged puzzler. Thanks to the solidarity of the Source engine, you get all the usual benefits. Like the other titles in The Orange Box, I didn't come across a single technical issue. Frame rates are solid - even on a very modest machine - and it still retains all the detail and level of finish you'd expect from a Valve title.

I'm not usually one to measure a game on the number of hours it takes to complete, as everyone's play through will inevitably differ - but I'll make an exception here. By anyone's reckoning Portal isn't a long game, weighing in at around the three to four hour mark. Those of you who do see this game from start to end though are in for a very special experience. From the voice acting and the mind-bending puzzles to the mysterious and somewhat eccentric tone that runs throughout, Portal is a short, sweet and compelling trip into an extraordinary offshoot of the Half-Life universe.

Reviewed by Martin Oddy for AceGamez (All Rights Reserved).