Threes is exceedingly simple. The game, which launches today on iOS for $1.99, blends the minimalist style of hits like Letterpress and Spelltower with the addictive number crunching of Sudoku. The result is a fantastically intuitive but surprisingly deep puzzle experience — and one of the best iPhone gaming diversions in quite some time.

As the name implies, Threes is all about the number three. The game takes place on a four-by-four grid, and each round starts out with a few cards already placed on the board, usually some ones and twos. These then need to be added up to create even larger numbers. This is done by squishing cards together, and all of the numbers in the game (outside of one and two), are divisible by three — two threes will create a six, which you can match with another six to make a twelve, and so on. Much of the challenge comes from the unique control scheme. You move cards by swiping left, right, up, or down, and doing so shifts every card on screen in that direction. Every time you swipe, a new card is added to the screen. As the board fills with cards and space becomes more limited, your choices become increasingly important.

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The goal is simply to get a high score, and Threes has that rare ability to put you in a trance as you swipe cards around and add numbers together with zenlike efficiency. Matching up two 192 cards is a particular challenge that will keep you coming back regularly. Threes' streamlined game design is complimented by its simple and playful art style. The cards themselves are essentially just numbers on a white rectangle, but they feature adorable faces that change as the cards rise in value. It's surprisingly fun to finally get a card wearing a pirate's eyepatch. That whimsical style comes courtesy of artist Greg Wohlwend, who previously worked on mobile hits like Ridiculous Fishing and Hundreds.

"The game has resisted complexity since day one."

Threes is Wohlwend's second collaboration with designer Asher Vollmer, as the two previously created iOS game Puzzlejuice, a fast-paced, stressful take on the typical word game. The basic prototype for Threes only took a single night to create, but the two have spent the last year refining the concept. And about half that time was spent on ideas that were ultimately scrapped, because they made the game more complicated than it needed to be — additions like power-ups and a monster character that ate numbers. "Threes didn't want any of it," says Vollmer. "It just wanted to be a cold mathematical brain game and we had to stop trying to add all these extrinsic features to it."

The result is a game with a strong core idea and very little fluff. Your time in Threes is really only spent doing one thing — adding numbers together to make bigger ones. Yet it remains constantly engaging despite its seemingly simplistic nature. It may be about numbers, but Threes is really a strategy game at heart. In order to get the really big numbers — the ones you'll need to inflate your high score and pose a challenge on the online leaderboards — you'll need to plan at least a few moves ahead. If you don't, you'll find yourself running out of moves very quickly.

"The game has resisted complexity since day one," explains Vollmer. Threes is destined to be your next mobile puzzle addiction, and the key to its deceptively straightforward design was a little restraint.