Epic Games’ iOS port of smash hit Fortnite has earned more than $15 million in its first three weeks on the market, says mobile analytics firm Sensor Tower. According to the company’s most recent report, Fortnite on iOS is now out-earning mobile heavyweights like Candy Crush Saga, Clash of Clans, and Pokémon Go on a weekly revenue basis, earning $6.4 million in the last seven days compared to Candy Crush Saga’s $5.8 million. Granted, those games have all been out for years, with Candy Crush Saga now a six-year-old mobile game. But each game on that list has remained at the top of the App Store for years, and they’ve each matured into massive and stable revenue-generators. Fortnite, at least for now, is leading the pack.

Unlike most mobile games, some of which allow you to pay money to advance progress. Fortnite sells only silly costumes, goofy in-game dance animations, and other cosmetic items, as well as a three-month $9.99 subscription service called the Battle Pass. And while these metrics for Fortnite seem eye-popping, they’re not that surprising. The game is a worldwide phenomenon due to a blend of smart timing, Epic’s free-to-play model, and the growing and innate popularity of the battle royale genre.

The genre was popularized by Playerunknown’s Battlegrounds, from which Epic borrowed the core concept of 100 players parachuting onto a deserted island, with the last person standing being the victor. But with Fortnite, a free game with a more accessible difficulty curve and more family-friendly and cartoony visuals, the genre really took off.

Fortnite’s mobile version succeeds in part because it is an exact copy of the desktop and console versions of the game, down to letting you keep your progress and cosmetic cache across platforms. That’s why players are comfortable spending money on the mobile version of the game; all the in-game skins, emotes, and other cosmetics carry over to PlayStation, Xbox, and PC. PUBG also exists on iOS and Android in a surprisingly stable state — it is made by Chinese gaming and tech giant Tencent — but progress does not carry over to other platforms.

According to Sensor Tower, when Epic lifted the invite-only hurdle for Fortnite on iOS earlier this week, a mechanism the developer had kept in place since the game’s March 15th launch, the company saw daily mobile revenue triple from around $600,000 to $1.8 million.

Sensor Tower estimates Fortnite has been downloaded 11 million times on mobile alone. According to Superdata Research, another game analytics company, Epic made about $126 million through Fortnite in-app purchases in the month of February. Mobile sales will surely boost that figure in the coming months.