From the Super Mario Wiki
Boo's Mansion is the home of some Boos in Paper Mario. Lady Bow and Bootler live here, as well as many other Boos. Mario needs to go through Forever Forest to get here. Down the road from Boo's Mansion is a windmill, Gusty Gulch, and Tubba Blubba's Castle.
The mansion consists of puzzles and traps that Mario must solve and get through to even greet Lady Bow. It has three floors.
In Paper Mario: The Thousand-Year Door, Boo's Mansion (though referred to only as "the mansion") is briefly mentioned by Bow, who appears (alongside Bootler) at Poshley Heights after the game is beaten. Apparently, it was a prolonged amount of time spent at the mansion that inspired Bow to take her vacation to Poshley Heights.
The concept of a mansion haunted by ghosts and filled with puzzles has been used in other Mario games as well, most notably Super Mario World and Luigi's Mansion. Also, the Ghost House featured in Dance Dance Revolution: Mario Mix looks extremely similar to Boo's Mansion, including similar Boo portraits and chandeliers.
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