It may be “free to play,” but that doesn’t mean it’s not making a ton of money. Fortnite was released on iOS platforms last month, and in the first three weeks, Epic Games has made a pile of cash just from in-app purchases.

According to Sensor Tower, a site that tracks app growth, revenue for the game has exploded, increasing 197 percent since Epic lifted the invite-only restriction. Total mobile worldwide revenue passed $15 million in only 20 days, with Epic pocketing $10.5 million of that total.

Even more surprising, Fortnite has quickly become the top-grossing game on iPhone, besting such venerable cash cows as Candy Crush Saga and Clash of Clans. Those games have been out for several years, but they’ve consistently maintained their top spots until Fortnite burst onto the scene. Globally, the game has racked up more than 11 million installs since its launch.

Fortnite: Battle Royale is a mode popularized by Playerunknown Battlegrounds, which drops 100 players onto a map in a giant free-for-all until one person is left standing. A steadily encroaching blue wall pushes everyone into a smaller and smaller area, and it results in a frantic mix of run-n-gun shooting and building defensive structures.

The in-app purchases don’t give any gameplay advantage; they’re mostly things like colorful costumes and victory dances for taunting a fallen foe. Epic Games is even looking for a new victory dance to add to the game with their #boogiedown contest.

Part of what’s driving the in-app purchases is the ability to carry over the progress and costumes you purchase to other platforms. So when you switch over to PC, PlayStation, or Xbox, you’ve got that new dance that you purchased on your phone the previous night.

Battle Royale is free to play, but you can also purchase the original Save the World mode, which pits you and some friends against endless hordes of zombies.

Some old-school gamers haven’t quite bought into the attraction of a multiplayer shooter on mobile, but there’s no denying Fortnite has become one of the surprise gaming stories of 2018. We found the game undeniably fun, despite the rather sloppy gunplay and mechanics that don’t quite realize their full potential.

Fortnite is Epic Games’ biggest success, and it may just be getting started. The revenue numbers are certain to grow in the future with the upcoming release on Google Play for Android platforms.