It's arguably the first First Person Shooter game, recreated as per the title as a shader. The enemies are Nazis, since few people would object to a game about

It's arguably the first First Person Shooter game, recreated as per the title as a shader. The enemies are Nazis, since few people would object to a game about an American soldier escaping a Nazi Castle and leaving a pile of Nazis in his wake. Or at least, that was the case in 1992.