The Bitlands is a dimension located beyond the yellow door in Super Paper Mario. According to The InterNed, the overworld music of The Bitlands is titled "The Open Plane", and the music in the underground area beneath the two red pipes is titled "Nostalgic Underground", due to it being an altered version of the classic underground theme from Super Mario Bros.
Chapter 3-1: When Geeks Attack
When Mario and co. reach the Bitlands, Tippi is kidnapped by Francis who then takes her to Fort Francis. The group finds its way through many obstacles, including a gigantic Koopa Troopa. Soon after, Mario discovers a makeshift fort populated by the remnants of the Koopa Troop. It is discovered that their leader is Bowser, who decides to fight Mario. He loses and joins Mario's party after a humorous conversation. The heroes then continue on to the Tile Pool. It is also possible to encounter a Pixl named Barry there. When the player talks to him, he says that they should return when they save Tippi. After Tippi is saved at the end of chapter 3-4, the player can return here and talk to Barry again. If the player does this, Barry will join them.
References to other Mario games
Names in other languages