Ninja Striker!

When you first start playing Ninja Striker!, you'll wonder how it's possible to fail. Your little ninja avatar tumbles through the air like a sworded wheel of death towards wherever you tap the screen, and you can string these somersaults together so that you never have to touch the ground.

You cut through Master System-era pixel-art ghosts and snakes and bouncing triangles almost without trying, accumulating a combo multiplier that only resets in the unlikely event that you take damage. You even cling automatically to walls and ceilings.

As if all this weren't enough, dotted around the stages are formations of arrowed discs that send your ninja pinballing around, hoovering up coins in a juddering, screen-shaking frenzy while you sit back and watch.

You encounter projectile-firing enemies a couple of stages in, which slightly increase your likelihood of being wounded, but even then you can easily make it to the end unscathed.

Then, after three stages, you get to the first world boss, a vaulting old ninja who hurls tornadoes, and after a painful introduction you realise that your infinite somersaulting ability is necessary for evasion as well as attack.

At this point it might finally occur to you to visit the shop, but when you get there you may be dismayed to discover that its wares - a range of items that permanently increase either your power or your health - are ruinously expensive.

You might just be able to afford the cheapest item - Straw Sandals - and the relatively cheap Woven Hat will only take you 15 or so goes to save up for by replaying earlier stages, but most of what's available will require you to plays tens or hundreds of times. And Ninja Striker! just ain't that fun.

Thankfully, you can buy enough coins in a £1.99 IAP to purchase almost everything in the shop.

As you make your way through the game the challenge increases, but not necessarily in ways that you can ameliorate with IAPs. For example, the arrow disks that brought you so much unearned wealth in the first half of the game start to turn on you, sending you backwards like an evil headwind or dumping you unceremoniously through the floor.

Your tap-to-somersault method of movement may be superhuman, but it's unwieldy in practice. Resisting the urge to stamp on your iPhone after your 20th failed attempt to guide your ninja through a narrow gap under a wall of downward-pointing arrows is an exercise in vein-popping restraint, made all the more bitter by the knowledge that no power-up can help you.

Ninja Striker! is a novel idea, and its presentation is authentically retro, from the pixel-art graphics to the sound to the menu text. There were no IAPs in Sega Master System games, but if there were this is how they'd have been presented.

But the game is incredibly uneven. It contains trivially easy sections punctuated by walls of eye-gouging difficulty, and IAPs that are unobtainably expensive with currency earned in the game but relatively inexpensive as long as you spend real currency.

If you can look past this, and you enjoy a challenge, it's certainly worth downloading. I'm not sure that it’s worth spending money on, however - if only because this doesn't necessarily make the hardest parts of the game any easier.