Blue Defense: Second Wave!

Blue Defense: Second Wave! isn't the first instance of aggressive force being used against an enemy as a supposed last resort, but in this case it's legitimate.

With colour-coded geometric foes aiming at the heart of your pulsing blue planet, you have no other option but to cut them down with bullets directed by a fingertip.

This is castle defence gameplay at its purest - the mediaeval setting substituted for a space age one, of course. It's a straightforward game that offers simple pleasures.

Unfortunately, it's far too simple. Lacking power-ups, upgrades, an experience system, or really any progression system aside from leaderboards and in-game achievements, Blue Defense: Second Wave! misses the chance to infuse its action with substance.

To fill a quick moment, it's ideal. Moving a finger across the screen adjusts an orbital gun that fires at enemies approaching from 360-degrees. Clever use of multi-touch allows you to use multiple fingers to fire a as many streams as you choose.

In sticky situations, you can resort to the powerful planet cannon by touching your planet to charge a shot which then zaps anything in the direction you slide your finger.

There's an accelerometer control option for the orbital gun (the planet cannon requires touching the screen), though it lacks the flexibility of the default touch control scheme. Given that enemies approach from every angle, toggling tilt controls means literally twirling your device 360-degrees.

The touch configuration isn't perfect since you often obscure the screen as you twist your fingers in frantic defence, but it's decidedly superior to the alternative.

Interesting enemies keep things fresh, which is vital given the game's surprising lack of depth. Beyond your basic orbital gun and planet cannon, there are no additional weapons to acquire. Furthermore, there are no defensive upgrades to unlock or stats to boost.

There's no progression mechanic to be found. Part of that is intentional, while the other part can be chalked up to missed opportunity.

The spotlight is squarely on Classic mode, which serves as a cure for high score fever. Here you're charged with protecting your planet for as long as possible, after which you post your score online and try again. It's great for filling the odd minute with pure action, even if it doesn't have depth to hold your attention for long.

Other modes provide more structure, in particular Level Select, in which you're able to tackle specific waves as featured in Classic mode. By breaking apart the waves, the game not only becomes easier to digest, but it makes earning in-game achievements more feasible.

Each wave comes with a set of three achievements, which you can earn by killing a certain number of enemies, completing the level without using the planet cannon, and finishing the stage without a single life lost.

While the decision not to integrate Blue Defense: Second Wave! with an existing social gaming network such as OpenFeint or Game Center limits the appeal, these achievements do provide some replay incentive.

A challenging Gauntlet mode also extends the game's length, though it doesn't necessarily enhance its value. These longer levels - the most advanced of which lasts an estimated hour - are no more exciting or pack anything that can't be found elsewhere in the game.

A structured campaign and a meaningful progression system could supply enough depth to transform this game from amusing to engrossing. As a quick distraction Blue Defense: Second Wave! succeeds, though it seems unlikely to survive beyond its 15 minutes of App Store fame.