Bethesda Game Studios made a lot of exciting announcements regarding the Fallout franchise during this year’s E3, and one of the most unexpected was the sudden release of Fallout Shelter, a mobile game that lets players design and run their own vaults.

Now less than a week after release, Fallout Shelter has already overtaken Candy Crush Saga as the third top-grossing iOS app according to App Annie, and it is currently the number one app overall.

Like Candy Crush, Fallout Shelter is free-to-play but earns money through in-app transactions. However unlike Candy Crush, those purchases are not strictly required to play or enjoy the game.

Why make Fallout Shelter?

Game Director Todd Howard of Bethesda Game Studios explained the motivation behind creating Fallout Shelter at the studio’s E3 press conference on Sunday.

“This game is going to be free, but don’t worry. There are no paywall timers,” Howard told a crowd of cheering fans at E3. “You don’t need an internet connection. You build something, it’s instantly built.”

He added, “The whole goal of this game was to do something we’d really want to play on our phones. Something that made us smile and had more depth than other things we were seeing.”

The game’s in-app purchases come in the form of lunch boxes that contain a random assortment of loot like weapons, armor, and other supplies. They also have the potential to add epic Fallout series characters to your vault like Colonel Autumn and Three Dog. Although lunchboxes can be purchased individually or in bundles, they can also be obtained for free by simply playing the game.

While the game has already been a huge success with Apple users, Bethesda upset more than a few fans by releasing Fallout Shelter for iOS only. The developer promised that an Android version will be “hopefully out in a few months,” but that did little to appease the fans who see the delay as evidence of a timed-exclusive deal between Bethesda and Apple.

Bethesda has mentioned no such deal, but the studio will have to come up with a better answer than “someday” if it wants to satisfy its rabid fan base.

Image credit: Fallout Shelter | Bethesda Softworks LLC