To the casual observer, Threes! might look like nothing more than a bunch of white tiles being slid around a grid.

But anyone who plays it for more than five minutes will know that slamming together two '192' tiles to make a mammoth '384' tile is about as challenging and satisfying as toppling any empire, winning any Grand Prix, or defeating any boss.

The basics of the game are pretty simple to grasp. There's a cramped and claustrophobic 4x4 grid that is littered with numeric tiles. When you swipe the screen, all the tiles are shunted over. Unless, that is, they're already up against the edge of the grid.

Three

With this restriction in play, you can slip two numbers on top of each other. So, you can squash a blue '1' tile with a red '2' tile, which snap together to create a '3' tile. Or you can squash two identically numbered white tiles (two '6s', for example), which snuggle up and get added together (to form a '12' tile, for example).

Your goal is to make as many combinations as possible before the grid inevitably fills up (a new tile is introduced every time you swipe) so that when you tot up all the tiles, you end up with a very high number indeed.

You can play it quickly or take it slowly and consider the ramifications of each move. It always feels good to combine numbers, but it feels even better when you've stacked the grid so you'll make a run of numbers - two '12s' turning into '24'; two '24s' meeting to make '48'; two '48s' getting up close and personal and forming a '96'.

It's a bit like Triple Town, though there are fewer things to combine here and far fewer bears. There's just something about Threes!'s deliberate pacing and the way those tiles slide into each other like a lock into a key that makes it feel so rewarding when you're on a roll.

Six

It's also frustrating when you do poorly, which is compounded by the fact that there are some random elements in the game that can halt your progress.

For example: you know what type of tile is going to arrive next and you know it will slide in from the same direction that you just swiped, but the exact lane it will show up in is random. That can have a catastrophic effect. Especially when the grid is getting crowded.

These random, chaotic elements are obviously required to keep the game fresh. It would be too easy, too predictable, and completely without tension if everything went according to plan. But when a promising run is curtailed by a string of unlucky moves, it can still kill your enthusiasm for the game.

Twelve

Threes! boasts a lot of style, with sharp, understated art from Hundreds visual designer Greg Wohlwend and a catchy jazzy soundtrack from Jimmy Hinson.

Plus, each tile has a unique personality, face, and voice, with many of the voices provided by noteworthy indie game makers.

All of this means that Threes! is a terrific iOS puzzler. It's compelling and compulsive, with a central mechanic that's effortless to pick up. The game contains sufficient hidden depth, though, to keep you opening it up again and again (just like I did while writing this review) to try and eke out a slightly higher score.