Wolfenstein is a series of World War II video games. It was originally created by Muse Software, before being revived by id Software. The first two games in the series, Castle Wolfenstein and Beyond Castle Wolfenstein, were developed by Muse Software and focused on stealth-based gameplay from a top-down perspective. Beginning with id Software's Wolfenstein 3D, Wolfenstein games became fast-paced first-person shooters, and are widely regarded to have helped popularize the first-person shooter genre. Further games in the series include Return to Castle Wolfenstein, developed by Gray Matter Interactive, Wolfenstein: Enemy Territory by Splash Damage, and 2009's Wolfenstein by Raven Software. After ZeniMax Media acquired id Software, including the Wolfenstein franchise, Swedish developer MachineGames became the series' primary developer. Wolfenstein games developed by MachineGames include Wolfenstein: The New Order, Wolfenstein: The Old Blood, Wolfenstein II: The New Colossus, Wolfenstein: Youngblood, and Wolfenstein: Cyberpilot.