Bethesda dropped a bomb during its Electronic Entertainment Expo tradeshow event when it announced its first major free-to-play game, and it looks like plenty of people took refuge in the the publisher’s Fallout Shelter.

After making its mid-month debut at E3, Fallout Shelter finished June as one of the top downloaded games for iOS, according to mobile-industry tracking firm App Annie. That’s a huge launch for a console- and PC-focused franchise, and it led to the game generating $5.1 million through its first two weeks on the Apple App Store. This has Bethesda and Fallout Shelter leading the way as an example of how traditional publishers can use their major properties to take home some of the $30 billion mobile-gaming business.

And Fallout Shelter likely owes its success to Bethesda pushing it across the invisible barrier that separates a commercial-as-a-game and something that can stand on its own.

“Rather than being mere promotion for Fallout 4, Fallout Shelter seems to be a genuine effort of Bethesda’s to craft an engaging mobile experience,” reads the App Annie report.

The smartphone adaptation is free-to-play, but it is missing some of the negative stereotypes of that business model. Bethesda promised upfront not to lock gameplay and items behind timers and paywalls. Instead, people can just purchase goodie bags filled with bonus items if they want to spend money.

And you can tell that Bethesda knew from the start that mobile is a place it wants to have a presence. Fallout Shelter isn’t just seem cash in, and the publisher is already prepping its next major release for the touchscreen devices.

“The company is looking to score again on mobile with an upcoming Elder Scrolls trading-card game for iPad,” reads App Annie’s report. “This sudden expansion to mobile appears to be the result of a growing recognition that mobile is an essential part of a well-rounded gaming portfolio.”