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 as having helped to popularize the first-person shooter genre. Further games in the series include "Return to Castle Wolfenstein", developed by Gray Matter Interactive, ' 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 ', ', ', ', and '.