It's hard to overstate just how good Threes really is. Touch Arcade's review deemed it a "perfect mobile game." Ken Wong, the lead designer of the Escherian insta-classic Monument Valley said Threes, ostensibly a rival for mobile mindshare, was "one of the most elegantly designed games since Tetris." Indeed, its central mechanic–sliding like-numbered tiles together to create higher-numbered tiles–is so utterly simple and exquisitely satisfying it seems like the type of thing that doesn't get invented so much as discovered. Like one stormy night, a cloistered indie dev woke up with a start, grabbed a notebook, and scribbled furiously until there it was, in completely and flawlessly formed.

That's not exactly how it went.

At various points in its development, the simple number puzzler included holes, flags, planets, atoms and arrows. For months it was bedecked in argyle, like your grandpa's favorite socks. A monster known as the "argoyle" featured prominently. In fact, a whole menagerie of beasts were developed for Threes, drawn and redrawn over the course of months. None of them made the cut. Even the central game mechanic changed dramatically, months into development.

>"We spent so much time on the monster. When we took it out, it just hurt."

After all of that work, it took some other developers all of an afternoon, give or take a few afternoons, to rip Threes off. One of those derivative games, 2048, all but eclipsed Threes, even though it borrowed only the superficial joys of the original while ignoring much of what makes it truly great.

The fact is, the simple magic of Threes came not from a lightning bolt of inspiration but rather a slow, steady grind. In the year-and-change it took to make, new features were added and iterated upon tirelessly, only to be scrapped. The developers had to kill their darlings, as the saying goes.

But in exhausting all those possibilities, they gained a certain confidence in their approach. It's what makes Threes feel like such a perfectly realized thing–the type of game you might find yourself revisiting not just a few months but a few new phones down the line.

Hard Enough to Be Played Forever

The Threes story began on December 3, 2012, when Asher Vollmer, then a developer for the renowned indie studio Thatgamecompany, sent a prototype of a sliding numbers game to some friends. It was an ugly, bare-bones affair: a bunch of big, chunky numbers crammed inside a four by four grid. Within the hour, Greg Wohlwend, a graphic designer who had worked with Vollmer on an earlier mobile game, Puzzlejuice, sent a mock-up with nicer graphics. By the next day he'd sent a few more.

The neon green and hot pink color scheme seen in one of these early screenshots would hang around for a surprisingly long time, but what's most interesting about this early prototype–just a single day into development–are the details that would weather the ensuing storm and find their way into the final release, namely, the typeface for the numbers and the shape of the tiles.

Threes transformed dramatically over the year that followed, in a collaboration that transpired mostly in emails between Vollmer and Wohlwend. The saga is available for anyone to follow. Last month, as the duo were figuring out what to do about the 2048 problem, they decided to "show their work," giving the world an intimate look into the making of their game. It takes the form of The Threes Letters, a compendium of correspondence comprising some 40,000 words in hundreds of emails. It's like the WikiLeaks of game design–a document dump that offers a fascinating peek behind the curtain.

From the start, Vollmer and Wohlwend were sure they had something good with their sliding numbers mechanic. It was, however, just about the only thing they were sure of. All else, from the core workings of the gameplay and the visual style, down to the tagline and animated GIF that would introduce the game to the web, remained to be worked out.

. Image: Threes

In Vollmer's early prototype, the game revolved around pushing numbers into a hole to earn points. Thanks to Wohlwend, the hole became a mouth, and the mouth became a monster. At some point around that time, the argyle showed up, bringing the argoyle with it. All the while, Vollmer was tinkering with the gameplay, adding things like walls that impeded certain swipes, and arrows, which demanded them. It never quite clicked. The game was satisfying from moment to moment, but missing the complexity that would bring true replay value. At this point, Vollmer and Wohlwend were trying to find the killer mechanic by adding stuff–piling on features. In the process, they were burying the basic gameplay that would eventually make Threes a success.

That February, Vollmer sent Wohlwend a dispiriting note. "Threes, in its current state, is not worth releasing," he wrote. The feed-the-monster mechanic made for a boring game. Each turn was low-stakes. The board could look the same on turn 20 as it did on turn one. There was little momentum to each session. "The moment you've A) learned the game and B) played with the game for a while–there is nothing left to achieve," Vollmer wrote. "My personal skill at the game has plateaued, and with nothing driving me to keep playing I see no reason to try to improve my score."

Volmer vowed to take the "spaghetti v. wall" approach to see if any particular permutation caught his attention, but his email was a blow to morale. "We knew that way of swiping was going to be really satisfying," Wohlwend later told me. "It just felt great. That's the toy of it. But I think the tricky part for us was finding out a way to make that work as a 100 percent rock-solid game."

The Breakthrough

That spring, Wohlwend and indie eminence Zach Gage released Ridiculous Fishing, which became a huge App Store success. Vollmer quit his job at Thatgamecompany. Nearly three months went by without any progress on Threes. The game, it seemed, was on life support.

In June, Vollmer sent another flurry of prototypes. The guys hadn't yet ditched the monsters, but one of the iterations got Vollmer thinking about a new approach, one that was fundamentally different from the feed-the-monster-to-cash-out mechanic they'd long been exploring. "What if you're just combining/upgrading Argoyles?" he mused.

visuals, including the beloved argyle version. Image: Threes

The idea for this "merge mode," as they came to call it, was one of many mini-revelations that ushered the game to its final form. "It was just a random what-if, sitting in a coffee shop," Vollmer explained later. "I plugged it in, and suddenly I just could feel it, like, 'Oh, this is really good. This is really interesting." Wohlwend worried the new mode was too simplistic, but Vollmer argued its case: "I actually like that it's so clean and pure. It's incredibly gratifying to have a system with such simple rules that you can play over and over again and constantly get better at. It's the reason I've been playing Drop7 for years without stop–this game feels like it has a similar skill curve and that's incredibly exciting to me."

The next step was finding a visual theme to bring merge mode to life. Wohlwend suggested one based on science and space, where protons would combine to create atoms, atoms would combine to create molecules and so on until you were creating galaxies. Ultimately, though, the monsters won again. The challenge became balancing the personality of the monsters with the legibility of the tiles. It all fell apart, after all, if players didn't know which tiles they could combine.

As it happened, the small breakthroughs that helped bring the game across the finish line came from two outsiders. Zach Gage, Wohlwend's former collaborator, suggested putting the monster faces on the bottom edge of the tile, leaving the tile's face free to show number. The move sidelined the monsters as such, but it dramatically simplified the appearance of the game board, opening up the option for a brighter color palette and finally letting them ditch the black background that had been a sticking point for much of the game's development.

development. Image: Threes

But Adam Saltsman, the designer behind the early iPhone hit Cannabalt, quickly identified a problem in their prototype. Using a "corner strategy," he found he could essentially get an infinitely high score. After some brainstorming, the group settled on a few adjustments that complicated Saltsman's game-breaking approach. For one, they tweaked the game so you could only see the type of tile you were getting next, not the number. They also made it so players couldn't easily force new tiles to appear where they wanted. Saltsman referred to the process as "topology management"–essentially finding the delicate balance between randomness and the tools players have to mitigate it.

With these final tweaks, the game achieved just that balance. After Wohlwend and Vollmer's extensive play testing and prototyping, Gage and Saltsman's incisive eye for game design brought Threes to a beautiful place. The final game manages an elusive thing Vollmer referred to occasionally in the development process as "foreverplay." The ceiling for experts is high. As of March, only six people in the world had made it to the 6144 card, and no one had yet fully "beat" the game. At the same time, it's enjoyable from the get go for casual gamers. Few games can so deftly serve this huge spectrum of skill level and strategy.

The game lived with a black background for much of development. Image: Threes

A Willingness to Kill Your Darlings

This Threes was significantly different from what Vollmer and Wohlwend imagined through much of the design process. It was smaller, more compact. It was about numbers, not monsters. But they took to this new identity, embracing the idea of a tiny game whose pleasures were whispered, rather than shouted.

Getting there was all about refinement. "Most of the time we spent was taking away ideas," Vollmer says. Even when you read through the year-long saga in its abridged, accelerated form, it becomes clear how fond the developers were of elements they ultimately axed. "It's definitely frustrating at the time," Vollmer says. "We spent so much time on the monster. When we took it out, it just hurt."

In the end, though, all those months lost in the woods were essential to finding the way forward. "The trap of game development you can get stuck in is constantly questioning your decisions and not having confidence. In game design, anywhere you can find confidence to hold onto is valuable," Vollmer says. "Because we have this year of work and testing under our belt, I can definitively say, 'No, that would be the wrong decision.'"

That confidence is what lets the duo dismiss 2048 as a "broken game." The problem, they write in the introduction to the Threes email log, is something they noticed about their merging mechanic early on.

"We wanted players to be able to play Threes over many months, if not years. We both beat 2048 on our first tries. We’d wager most people that have been able to score a 768 or even a 384 in Threes would be able to do the same using the fabled 'corner strategy.' You probably could too! Just try tapping 'up' then 'right' in alternating order until you can’t move. Then press left. You may not get to a 2048, but you might just see your highest score ever."

The reason Vollmer and and Wohlwend can say this authoritatively is because Threes, too, was a broken game for all those months. Fixing it involved a colossal amount of trial and error, tweaking, iteration and excision. It was a journey with countless dead ends–a process that perhaps really was more about happening onto the right path than blazing a new trail outright.

The feeling of finally making that discovery? Relief. "There's nothing that describes it better." Wohlwend says. "You're constantly worried that you're not going to find it. It's not like, 'God I'm good at this.' It's 'oh thank God I found it.'"