Nintendo has crossed over $1 billion in revenue from its mobile games for Android and iOS according to a new report from SensorTower — largely carried by the popular free-to-play Fire Emblem Heroes, which was released in 2017. It has since gone on to earn $656 million, more than every other Nintendo mobile game combined.

Fire Emblem Heroes is a “gacha” game, where players spend in-game items to randomly pull new characters with different abilities, items, and stats. Unlike games like Fortnite, where players can buy new character skins outright or earn them over time through a Battle Pass, everything in Fire Emblem Heroes is distributed randomly: you might get the rare character you’re hoping for on your first pull, or spend hundreds of dollars on thousands of pulls to never get what you want.

Developer Intelligent Systems constantly adds new and increasingly powerful characters (including some that are seasonally themed and only available for a limited time), which helps keep a constant stream of revenue.

That’s in contrast to Nintendo’s first mobile experiment, Super Mario Run, which promised a reimagined Mario experience specifically designed for mobile. The game was okay, but didn’t play very similarly to classic Mario games. It also only offered a single upfront $9.99 purchase instead of the constant revenue of games like Fire Emblem Heroes or Animal Crossing: Pocket Camp (the second top earner), leading Nintendo to brand it a “disappointment” despite hitting over 200 million downloads.

It also helps that Fire Emblem Heroes is a good game, translating the tactical combat of the mainline console games to a smaller, quicker format to play on the go. It’s that enjoyable gameplay that makes up the other half of Fire Emblem Heroes’ success, and is something that games like the Candy Crush-copying Dr. Mario World (Nintendo’s least profitable mobile game, by no coincidence) lack.

Nintendo is already building on that Fire Emblem success, too, with its latest mobile game: Mario Kart Tour. Like Fire Emblem Heroes, Mario Kart Tour takes a recognizable property (in this case, Mario Kart, which is arguably even more popular than Fire Emblem), translates the gameplay to a mobile-friendly format, and adds in gacha elements on top for a nearly endless revenue stream.

It’s a bet that’s already paying off: Mario Kart Tour was the most downloaded game on iOS last year, and it’s already surpassed Super Mario Run in revenue, despite only having been out for a few months.