If the PSP has proven itself as good for something, it is as a platform for quality puzzle games. From Lumines and PQ to Mercury, Sony's handheld has been a home to a select number of exclusive and innovative brain-taxing titles. The sleek slab of circuitry may be dominated by clunky PS2 ports and misguided action spin-offs, but the elite roll call of puzzlers it has seen have been almost consistently brilliant, and perfectly pitched for public transport gaming.

CRUSH is the latest contender to vie for a place on this list, and it certainly ticks all the boxes that qualify it to join the catalogue of intellectual pursuits for the mindful PSP owner.

At its core its gameplay is inventive and original. Cast as Danny, a cynical insomniac who, through his desperation to be cured, finds himself at the mercy of a stereotypically anarchic professor, you must navigate your way through a hi-tech therapy known as CRUSH. The revolutionary treatment which, metaphorically, is realised as some kind of can-opener-whisk for the inner psyche, requires the nervy hero to navigate a series of small, but intricate platform-based levels.

Hanging in some infinite space like the most taxing sections of Super Mario Sunshine, each level in CRUSH consists of a tight cluster of 3D platforms. While physically close, they are often impossible to navigate, thanks to a selection of impenetrable walls and ceilings, or Danny's rather uninspiring attempts at acrobatics.

However, with a firm stamp of the foot, triggered by tapping the left shoulder button, the level can be flattened down into two-dimensions, realigning the platforms in newly negotiable ways. This process is best explained through example. If the camera, which you can snap into place around or above the hapless hero, is viewing the 3D level from one side, the game will be flattened from the side on. What this mean is that a platform in the background previously out of reach will suddenly be as close to you as the one under Danny's feet.

If this still isn't clear, imagine going out into a long, straight stretch of busy high street, and looking directly down it into the distance towards a supermarket. Then imagine what would happen if perspective were taken away, and the bustling street was made completely flat to your viewpoint. Suddenly that shopping trolley that was 200 yards away would be within arm's reach.

As soon as you realise that CRUSH has dexterously handled almost all the potential that this simple concept offers, moving through the game quickly becomes thrilling and perplexing. Crushing and uncrushing, and swinging the camera round to flatten from a different angle rapidly becomes second nature, letting you move through areas with a carefree disregard to physics and commonsense.

The mark of a good control scheme is one that is easy to forget, and while guiding Danny through his neurosis, the button pressing soon becomes a distant memory, letting you lose the real word about you as you weave through the numerous levels.

Levels are completed when enough of the marbles that litter the play area are collected, which Danny scoops up as he wanders about in his dressing gown and slippers, looking and behaving like a virtual version of The Hitchhiker's Guide to The Galaxy's Arthur Dent.

Quickly a host of various enemies, bonuses, obstacles, and functional ability blocks appear, which can generally be moved through the crushing system with a simple push. Some, like the giant spheres that are introduced very early on, can be used as moveable platforms to clamber onto, while others boost the length of Danny's jump, or even just add to your level ranking.

Few puzzlers on the PSP pose as much a challenge

While all these extras are thoughtful inclusions to CRUSH, in some levels there is such a mix of different gameplay elements it can feel fairly overwhelming. Still, an overenthusiastic approach to variation is better than an understated one, and throughout CRUSH there are nice touches and signs of a loving development process. The occasional cut-scenes are full of appeal and the consistency and solidness to the game world is a must in the patience-stretching world of the puzzle game.

Occasionally the 'zany' visuals can get a little tiresome, and rarely touch on anything we haven't seen elsewhere with more panache. Stylistically the game is competent, and tidy enough to do the job, but the graphics rarely creep anywhere near being striking in any way.

Other niggles are few and far between, but as Danny progresses the spiralling difficulty does lose touch with the logic that makes CRUSH so enjoyable for the vast majority of the game. Towards the end of the later levels, crushing from one dimension to another quickly deteriorates into a demoralising process of trial and error. A good puzzle game should make you feel like you are sweeping through with the dazzling power of your intellect; while at times CRUSH makes you feel like you are bluffing your way towards an impending dead end.

It is important to emphasise that a huge amount of CRUSH is brilliantly playable and consistently enjoyable. It is bursting with character by puzzle game standards and, even considering its shortcomings, is still a PSP release well worth picking up if you are looking for something fresh and stimulating.