Crusader Kings II is a grand strategy game set in the Middle Ages, developed by Paradox Development Studio and published by Paradox Interactive as a sequel to Crusader Kings. It was released for Microsoft Windows on February 14, 2012. An OS X version, Paradox Interactive's first in-house development for the operating system, was released on May 24, 2012. A Linux version was also released on January 14, 2013. It has been Paradox's second most successful release to date, after Cities: Skylines, with over 1 million copies sold.