Endless runner Crossy Road has only been out on iOS for a month, but developer Hipster Whale is already celebrating over ten million downloads. It's a large milestone for the indie Australian developer, and the number only stands to grow as the game launches on Android this week.

The success of Crossy Road can mostly be attributed to its simplicity. Straightforward controls make it possible to get in a quick game no matter where you're at, and a score-chasing leaderboard encourages constant competition between friends. The charming, voxel aesthetic and easily unlockable characters just add to its minimalist appeal.

To find out more about what lies behind Crossy Road's hidden secrets, what it's like to be spotlighted by PewdiePie, and future plans for the game, GameSpot spoke with the duo behind Melbourne-based developer Hipster Whale: Matthew Hall and Andy Sum.

Why the game is free

Crossy Road is free to download and doesn't put up paywalls that prevent players from continuing. Additional characters are unlocked by playing, though players have the option to purchase them with real-world money to speed up the process. For Hall, the game's model was a combination of experimentation and learning from the mistakes of others.

"There's very little precedent for games that are this friendly. There was a game called Punch Quest, which I really loved, that had a really friendly approach to monetization," Hall said. "You could pretty much play it as long as you wanted without paying anything. It got an astounding number of players but very few people actually bought into it. It was almost like a warning to people not to try this way. But as a consumer, I don't really like consumables that much. I don't really like buying coin packs. It's an experiment to see if other people felt the same as me... I don't think [Crossy Road] would've gone viral if we had put paywalls."

"I think that's one of the reasons why people felt comfortable sharing it with their friends," Sum added.

The model has evidently paid off. It only took a month to hit the 10 million download milestone on iOS and last week saw the app's launch on Android via the Amazon app store. The Google Play store debut is set to happen on January 8 next year.

"I think we made our investment back day one, or day two. Since then, everything's been going up and up," Hall said.

The best character

Sales come from the purchase of in-game characters, which can also be unlocked through playing. With more than fifty characters currently available, Sum and Hall have read a lot of online debate regarding which one's the best.

"There's characters that make it harder, but none of them make [the game] easier," Sum explained.

"I was particularly inspired by Dota 2's model for payments. That anyone could play the same game but it has cosmetic changes. I think that was really important for the high score thing as well. If we had characters that did change the game then you wouldn't be able to compare your scores with your friends."

"It then becomes this personal thing about 'Who's your main character?'. Everyone's got their favorite; I always play Lucky Cat," Hall said.

What about characters that change the game in reportedly "strange" ways, such as the Forget-Me-Not? Do they mean anything?

Hall said, "Forget-Me-Not was a game that I really loved by a Melbourne designer called Brandon Williamson. It's one of my favourite iOS games, and it sort of went into obscurity. But I really loved the character, and I wanted to help him out, so I petitioned him to let us put the character in the game. The little pink things are from his game. The more you collect, the more it changes the game."

"He's one where it gets harder and harder," Sum explained. "As you collect a few [forget-me-nots] it starts to flash random colors as you pick them up, and annoy you. After a while, a ghost spawns and covers the screen. We kind of thought if two people had that character, they might talk amongst themselves and be like, 'What's your highest score with Forget-Me-Not?' because he's so hard to play with."

The Forget-Me-Not character also talks about the game it originated from "during the ad break."

Illuminati

Unlocking in-game secrets is a big deal in Crossy Road.

"We thought it would work, but it's been really popular so we're going to double our efforts for introducing that kind of stuff," said Hall.

Sum described the community's reaction to in-game secrets as "really interesting" and helpful in encouraging players to discuss the game further. For the most part, all of the game's secrets have been unearthed already, although Hall says that narratively, there are still plans to implement more discoverable content.

"We want to have secrets and achievements not only globally, but per character, and then grow the game narratively through that system. For example, the Mad Wizard and the Dark Lord are actually narratively mortal enemies, but no one actually knows this. The idea is that if you unlock both, then you have a chance of them fighting one another, and that will then unlock an achievement, and that's the ending for those characters. We have lots of ideas for that sort of stuff that we will slowly thread through in time. Secret characters have actually been hugely popular," Hall revealed.

The PewDiePie effect

When popular YouTube personality PewDiePie uploaded a video of himself playing another game Sum had worked on called GAME OF THE YEAR: 420BLAZEIT vs xxXilluminatiXxx [wow/10 #rekt edition], the exposure led to what Sum dubbed "a pretty busy couple of weeks." Several people who played 420BLAZEIT went on to discover Crossy Road.

"Crossy Road was already doing so well, but it definitely made a difference. We were already near the top of the charts at the time," Hall revealed.

"I think Crossy Road was able to stand on its own anyway," Sum said, but adds that PewDiePie's video led to an additional "thousands of downloads."

Future plans

The response to the list of unlockable characters has prompted Hipster Whale to continue delivering more, similar content to players, as well as offering a few deviations from the formula.

"We will experiment later with having characters that change the game. A lot of people are starting to play the game as speed runs. For example, how quickly can you get to 100. And we want to put in some characters with their own leaderboards. So we'll be doing that, if not this update, then the next one," Hall said.

Also on the agenda is the implementation of global achievements, character-specific achievements, and giving the game a narrative backstory. While Hipster Whale has plans to move onto a more long-term project, Crossy Road remains their top priority.

"It'll be some time before we can start working on other things," Sum assured.

"I like making different things each time. The idea that we're talking about is quite different. So we'll see." Hall said.