Geometry Wars may have just about succeeded in being Xbox LIVE Arcade's unofficial mascot game, and can certainly take some of the credit for the success of the service. The twin-stick shooter's popularity has proved that one of the oldest genres can still be very profitable, and has set the standard for Sony's PlayStation Network - a service which has been dominated by downloadable grid-based shmups.

This is all great news for hardcore gamers, and a perfect opportunity for casual players to learn that fantastic releases need not carry a £40 price tag and a fist full of overused conventions.

Nucleus is the latest 2D shooter to join PSN's catalogue of games, and mixes touches of previous Network releases Blast Factor, an intense twitch game, and plodding plankton sim flOw, with a more than a hint of Geometry Wars' gaudy bullet spraying madness.

At first glance it looks like a fairly basic grid shmup, but playing it reveals two qualities that go against the grain of what traditionally defines a shooter: complexity and lack of pace. Normally games of this genre are frantic and straightforward, concerning the player with concentration and skill rather than a weighty manual. Most shooters do have intricacies and nuances of course, but Nucleus even exceeds the more elaborate genre leaders Ikaruga and Gradius V in terms of rules of engagement.

Set in a microscopic world filled by a viscous liquid that creates an unusual sense of playing in slow motion, this is still a punishing challenge once the initial levels are out of the way. The basic mechanic focuses on building walls and defences from benign cells that fill each one-screen level. Most of the cells can be nudged into place with a quick 'squirt' from your craft's rear thrusters, and stick together to form spawn like masses to shield you from attack.

Nucleus' complexity makes it stand out from the crowd

Various organisms and viruses attack you from all sides, and many eat away at the shield as you build it, releasing tiny proteins that drift erratically around you. These proteins are one of the game's power-ups, providing you with smart bombs every time you collect a certain amount, which can be detonated instantly or aimed like a homing missile at the toughest enemies. Other bonuses boost the firepower of your main weapon, and a small directional tractor beam is available, to allow you to tow clusters of barrier cells and position them accurately.

The sluggish pace and detailed rule set mark Nucleus out from its contemporaries, and are at first rather daunting and frustrating. Quickly, however, you will find yourself lost in Nucleus' miniscule world, and despite regularly testing your patience, it is nonetheless a brilliant shooter that should be praised for originality.

The game's chosen topic means that by its very nature much of Nucleus is shrouded in rather mute colours, and all too often the enemy sprites become lost in the swirling background that regularly matches the colour of your foes, resulting in some infuriating deaths. A healthy dose of neon bullets do pepper many levels though, bringing some well-needed vibrancy to an otherwise subtle graphical palette. Sadly the low, rumbling soundtrack has even less sparkle than Nucleus' colour palette, to the extent that it feels like it needn't be there.

Nucleus is a curious game that will seem a little uncomfortable to some factions of both the shmup fanboys and casual players. Still, it is a great piece of software that manages to offer plenty of variation through well-designed level objectives, and at a price you can afford with pocket change, it's probably worth checking if Nucleus is for you.