From the Super Mario Wiki
Twilight Town is a perpetually twilit town in Paper Mario: The Thousand-Year Door. While Mario and his friends were searching for the fourth Crystal Star, the residents here were turned into pigs by Doopliss, who eventually stole Mario's identity and tricked his friends (the team was unaware that the Mario they were with was actually Doopliss) and caused trouble here. However, Vivian the Shadow Siren joined Mario's team to help Mario break the curse and get his appearance back, abandoning her sisters in the process.
A small section of Twilight Town called Twilight House, a reoccurring battle course in the Mario Kart series, is a large mansion suspended above the Twilight Trail. The battle course first appeared in Mario Kart DS and was later brought back as a retro battle course in Mario Kart Wii.
When Goombella tattles on the gatekeeper (who makes sure nobody goes past the gate to Twilight Trail which leads to the Creepy Steeple), she says that there is "a gatekeeper" in every town and if there wasn't, the game would be much easier to beat (because the player wouldn't have to deal with the mayor or get past the gatekeeper, they would just run to Doopliss). The mentioning of the "game," which refers to Paper Mario: The Thousand-Year Door, breaks the Fourth wall.
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