Shoot-em-ups are at the very core of what video games are all about. From Space Invaders to Galaga, Raiden and beyond, their simplistic design and addictive gameplay have an enduring appeal that keeps players coming back for more. Polish developer Infinite Dreams stepped into the fray way back in 2004 with its title Sky Force, launching on mobile devices and later soaring onto consoles with revamped graphics, new features and a more refined experience. It's been quite a journey to this point, and now Sky Force Anniversary is ready for action on the Wii U eShop, hoping to bring some life back to the console's store.

It's no slouch either, showcasing all of the fast-paced arcade gameplay you'd expect, with bright visuals and an immediate jump into battle before a single menu even appears. You take control of your aircraft as the main title drifts onto the screen; it's a striking and surprisingly cinematic way to show a sense of freedom and kick things off early, before a smart plot device comes into play. Your first burst of gameplay is cut short, your aircraft is stripped of all its upgrades, and you're back to square one. No, it's not Samus Aran flying the plane, but you'll still need to take revenge and build up your arsenal across a variety of different levels.

There are nine stages in total, but simply reaching the end isn't enough to progress. You'll need to fulfil a handful of extra missions to earn enough medals to unlock new areas, meaning that it's practically a requirement to revisit levels and try and fine-tune your performance on each one multiple times. From saving civilians to defeating a certain percentage of enemies, many of these challenges are incredibly difficult – if not outright impossible – to complete with your standard loadout, and so the upgrade system comes into play.

Between missions you'll be reminded to visit the hangar and spend your hard-earned stars on ship upgrades. You can purchase new weapons to upgrade your firepower, increase your maximum health and tilt the odds further in your favour. It should be noted that the upgrade process is actually fairly slow, and initial improvements to your base damage and health are incremental at best. Later upgrades like lasers and missiles will require repeated playthroughs to save up enough stars, which ties into the challenge system as you'll never be able to earn all the extra medals on your first try with pea shooter cannons and paper thin armour. There's a constant cycle of replaying missions to unlock stars to upgrade your ship so you can replay missions to earn medals to progress. This is acceptable due to the strength of the actual gameplay alone, but it certainly won't appeal to everyone.

While this gets fairly repetitive, clearing all the challenges on a stage will unlock a harder difficulty where enemies take more hits and deal more damage – no new enemies or patterns unfortunately - which perfectionists can sink their teeth into. There's also the option of bringing in a local co-op player to help out (no online element unfortunately), and with a wingman on the same screen it makes for an enjoyable multiplayer romp that takes the pressure off flying solo. In addition, there's also a new tournament every weekend, which are bullet hell style scenarios where you evade an onslaught of turrets while your weapons are totally disabled. As you're tasked with rescuing civilians and collecting as many stars as possible before getting completely wiped out, it's a nice change of pace and a fun challenge that focuses on skill rather than firepower.

Our controller of choice throughout the game - the Wii U GamePad - is used as a second screen only, but we found that it actually went totally blank every once in a while for no apparent reason. We couldn't reliably recreate this odd glitch so it may have just been an issue on our end, but still a weird anomaly.

The controls themselves are smooth and responsive across the board, as is the game's general performance. Visuals are undeniably eye-catching, though there isn't a whole lot of diversity when it comes to the different environments. As the game is set across a series of tropical islands there's a lot of ocean and jungle with only the occasional power station or enemy camp to break things up, and more than a few re-used assets. That being said, everything is always nice and clear and doesn't hinder the gameplay at all, even in different weather conditions. In fact at times the perspective and 2.5D nature of Sky Force make it seem as though you're looking down on a miniature diorama, albeit one that's frequently filled with explosions. It's pretty to look at, but you'd better get used to seeing the same level over and over as you work on earning those medals. The soundtrack is similarly repetitious, with a few basic tunes and voice clips that we ended up muting so we could listen to podcasts while grinding for stars.

That's really the crux of the experience - grinding. Learning enemy patterns and focusing on every single bullet is a vital part of earning medals and surviving later areas. If you die in the middle of a level you're sent back to the beginning, but as levels only take about 5 – 10 minutes to clear and you keep all the stars you've earned, it isn't a huge inconvenience and doesn't feel like wasted effort. You simply upgrade and try again, and again, and again, until you whip through all the stages on offer in a couple of hours. Failure isn't always a matter of skill either, it's just down to not having the right tools for the job until you spend time upgrading, so the difficulty curve can be pretty frustrating. This may be a smart device legacy, but overall we found ourselves getting drawn back and enjoying the loop.

Conclusion

While Sky Force Anniversary doesn't do much in the way of re-inventing the shoot-em-up genre, it's a testament to just how well this style of game still holds up today. It's addictive, it's satisfying, and it encourages you to hone your skills to score perfect runs across a limited variety of levels. There's a good amount of replayability here as a result, though that doesn't make for a great deal of diversity or exciting set-pieces to look forward to. The mechanics are sound, but feel weighed down by the need to grind in order to progress, along with a serious dose of deja-vu from the environments.

Still, at its low price this is a flight well worth taking, even if it only lasts a couple of hours. If you're happy to grind and enjoy a well made shooter, it's a welcome option on the Wii U.