In this particular case, it's entirely fitting that the "Bottom Line" is displayed (at the time of writing) at the top of the review. In a game where up and down are virtually interchangeable, where beginning and end can be practically one and the same, it's pretty clear that the laws and dimensions of the physical universe as we know it have lost all relevant meaning.

Welcome to Portal.

Since the final results came first, you'll have noticed its high score right away, so if all you want to know is that Portal is easily one of the best games on the market today, feel free to skip directly to the exit and on to the nearest store to pick it up. But if you'd rather take the long route to explore the matters of significance, follow along and I'll tell you why Portal is also one of the freshest, most unique gaming experiences ever offered, though one that's over a little too quickly and may not be for everyone, particularly those whose tastes (and abilities) are restricted to more conventional adventure game fare.

Portal is not a new game, having first been bundled in Valve's The Orange Box for the PC (the version being reviewed here), Xbox 360, and PlayStation 3 late in 2007, along with the previously-more-celebrated Half-Life 2 games. At that time, the action-oriented nature of this collection allowed Portal to slip largely unnoticed through the cracks of the adventure community's attention. Fortunately, its overwhelmingly positive reception has prompted Valve to release a standalone version of the game, casting a well-deserved spotlight on it once again and making it a much more appealing option for those only interested in the largely cerebral Portal experience.

But what exactly is Portal? That is a question not easily answered, and even less easily after a glance at the game's screenshots. A picture may be worth a thousand words, but in the case of Portal, a thousand could be spent explaining why the game is NOT what it first appears. Yes, it's played from a free-movement, first-person perspective; yes, a reasonable amount of dexterity is required; and yes, you spend most of the game holding and firing a gun. But no, it's not a shooter by any stretch of the imagination. You'll also make your way, often running and jumping between seemingly impossible heights, from one end of nineteen sequential, self-contained levels to another. But it's not a platformer. At times it resembles a simple physics simulator and at other times a pure puzzle game, and yet often much more than either one alone. The inescapable fact is that it's really unlike any other game, refusing to be pigeonholed into any particular genre. So can it be called an adventure? Sure, sort of. And sort of not. What is it? It's Portal.

The game begins, with no cinematic introduction at all, as you awaken in a small glass room inside a clinical laboratory of some kind. Before pondering too deeply about who and where you are, you're addressed by the only companion you'll have throughout the game, the computerized female voice of GLaDOS, "Genetic Lifeform and Disk Operating System" of Aperture Science. GLaDOS reveals that you're the latest test subject of Aperture's Computer-Aided Enrichment Center, and that you're about to begin your trials.

Not surprisingly, the tests you undergo involve mastering the use of portals. Any pair of portals allows you to enter one and exit the other, regardless of distance or even orientation. Floors, walls, platforms, and ceilings are all fair game, though only some (easily recognizable) surfaces can host a portal. It's an utterly simple concept, but not so easy to apply in practice, as it first involves un-learning the intuitive physical boundaries we're used to. It just isn't natural to hop into a portal entrance at your feet and come tumbling out of the ceiling above you through its partnered exit point, but in Portal you'll be asked to do precisely such maneuvers with regularity.

To help players adjust, the early levels are quite easy, limiting manual portal control until you've mastered the basics. Even when you obtain the portal gun, initially it has the ability to set only one portal, with the other already strategically located in the test level itself. Also in these early stages, GLaDOS gradually introduces the various other obstacles you'll encounter. These include the likes of energy emitters, whose discharges need to be directed into inconveniently-placed receptors, movable cubes and weight-sensitive floor switches, particle barriers that only you can pass through, acid pools, moving platforms, a few timed doorways, and both mounted and portable turrets. To further nudge you along, there are symbols at the start of each level, hinting at the various tasks the stage will require.

Eventually, you acquire the portal gun upgrade, which gives you full control over both portals, and that's where the challenge begins to ramp up, although still fairly gently to start. With this newfound freedom you'll learn the more complex techniques, such as shooting new portals while falling in mid-air and projecting yourself through portals using the natural momentum of gravity. This skill of "flinging" – described in layman's terms by GLaDOS as "speedy thing goes in, speedy thing comes out" – is vital to getting through the later levels, but by then you should be plenty comfortable with both the concepts and mechanics of the game.

Controls in Portal couldn't be much simpler, utilizing the standard mouse-WASD keyboard combination of most action games. There is no inventory, just the ability to pick up and carry a single item in front of you until you release it. The dual portal gun is fired using the left and right mouse buttons, and the portals are coloured differently so that you can easily remember which you need to place next. There is only one movement speed, a slow run that is more than enough since any significant movement is reliant upon portal placement, not fleetness of foot. Jumping is as easy as a tap of the space bar, and while you'll do a lot of leaping throughout the game, relatively few require precision timing or skillful physical maneuvering. There are a handful late in the game that should prove a challenge, perhaps a little out of sync with the game's demands otherwise, but these are certainly the exception rather than the rule.

In fact, the "action" quotient in Portal is fairly minimal overall. Despite its superficial shooter-like comparisons, the whole point of portal use is to defy physical obstacles – to manipulate or bypass them entirely rather than overcome them through speed or force. The exercise, then, is one of tactical planning through a series of environmental puzzles, requiring far more thought than reflex. You can die, but not often and not easily. Even the turrets do minimal damage, generally allowing you loads of time to duck out of harm's way, instantly restoring you to full health. When you do die, you're automatically returned to a recent checkpoint. You can save manually at any time, but you'll rarely feel the need as the game auto-saves frequently anyway.