Free-to-play game company Kabam acquired San Francisco-based game studio Phoenix Age, adding iOS role-playing games to its portfolio.

Phoenix Age is the maker of ‘Underworld Empire’ and ‘Castle Age’ which both feature in the top-grossing charts for iOS. The company is currently developing an as-yet-unnamed third title.

“Phoenix Age’s hit games and world-class talent further solidify Kabam’s leadership role in the free-to-play games space while expanding Kabam into new genres,” said Kent Wakeford, COO of Kabam.

Edwin Shew, co-founder of Phoenix Age, said joining Kabam gives the company “expertise, experience and infrastructure to scale our games to a global level we couldn’t achieve independently”.

Financial terms of the deal were not disclosed.

Kabam has successfully transitioned from generating most of its revenue from Facebook games a few years ago to making much of its money through selling virtual goods in its free-to-play mobile games.

The US-based company has acquired six game studios since 2010 and has expanded into publishing with agreements with prominent game developers in Asia such as Game Reign, WeMade, and Gfan.

Four of Kabam’s games have grossed more than $100 million to date, and its games are often among the top 25 grossing games on iOS and Android.

The company generated $360 million in revenue in 2013, and forecasts this to rise to $550-650 million in 2014. Kabam CEO Kevin Chou has said in the past that he expects an IPO by the end of 2014.