Quake (video game)
The levels are grouped together into different episodes, and each episode has a different setting. Most levels look very Gothic or medieval, but others look more futuristic, like science fiction. There are 6 or 7 levels in each episode, but every episode also has one secret level. There is also one level that is not inside of an episode, which is the final boss level. There are four episodes in the game, and they can be finished in any order the player wants. The player can't get to the final boss without beating all four episodes, though.