You might think with the release of the G.G. Series Collection+, Genterprise and Suzak would be done releasing budget games for the DSi. After all, the collection includes all the G.G. Series DSiWare releases to-date, plus another 15 games never released to the DSi Shop, but apparently it's time for sequels as one of the first games, Z・ONE, now has a follow-up you won't find in the aforementioned retail release. It's pretty much like the progression of Gradius to Gradius 2: the game structure is the same and the rules are the same, but there's a couple of extras and the difficulty has been increased.

Just like the first game, you control a Vic Viper-styled spacecraft flying through the corridors of some kind of alien installation in gameplay more reminiscent of Konami's earlier shooter Scramble, since the focus is more on navigating narrow passages than shooting enemies. The two biggest changes are a choice of two different weapon pod types and three different zones of increasing difficulty – alpha, beta and gamma – each of which it seems, must be beaten to unlock the next.

The original weapon pods are back and work the same: when you move, the guns will shift aim to the opposite direction of movement so you simply press and hold the fire button to lock the direction in place. It takes a bit to get used to, but you can adjust quickly enough. The second weapon type sees your forward- and backward-movement shift the pods around the top and bottom of your ship in a semi-circle from forward to rear allowing for a wide range of firing arcs in two directions. We found this an easier weapon scheme to get to grips with and liked the ability to hit targets at two elevations at once and we like the novelty of choosing weapons before starting the game.

Although alpha zone is subtitled "Easy" that's not really the case and you'll find that there's a slightly stiffer challenge on offer here than in the first game with this name. The high-speed zones feel much faster and the regular gun emplacements seem more plentiful, as are their bullets. End level challenges filled with respawning enemies also pose a greater threat as the screen fills with bullet patterns to dodge in the event you slack off the pace of destroying them. You only start out with two ships this time and must complete a level without dying in order to earn an extra life rather than simply scoring points. After all lives are lost you can choose to retry the current level, but with the penalty of having your potentially record-breaking score zeroed out you probably won't want to. No self-respecting gamer would do that anyway, right?

Another new feature is the controls. A second button has been added to launch your weapon pods out in front of your ship, divorcing ship control from the firing arc (where they'll end up is indicated by green targeting reticules that also help indicate what direction you're currently firing in). This can be handy for hitting enemy guns before they can target your ship, and also allows you to freely manoeuvre without compromising on hitting every available target. The downside is that in order to change your pod aim you'll need to recall them, aim, and then launch them again, and of course you'll lose their protective bullet-blocking ability. In addition, you can freely remap the buttons as you like – a rarity in this series of games.

As with the first game this isn't anything too special, but for a budget release it looks pretty nice and certainly would have impressed in arcades in the mid-1980s. Parallax scrolling is used again for depth of view and the bricks that compose the narrow corridors you're flying through have pulsing colours that seem to complement the beat of the techno soundtrack. For shooter fans it's a nice bit of fun you can play for five or ten minutes and enjoy; the recording of separate scores by weapon type and level will keep you coming back to better that score.

It's good to see Genterprise continuing to publish Suzak's budget game series beyond the retail collection of their previous releases - we certainly think it's one of the more worthwhile third-party efforts on the DSi. Who knows, perhaps in a year's time we'll have another collection of cheap and cheerful downloads to look forward to at retail!