Alternative Castle

Appearing rarely in personalised copies, the Alternative Castle is an anomaly which causes the appearance of Princess Peach's Castle to change drastically. The music can apparently be a 'slightly-off' version of the regular castle theme, or modified versions of pre-existing songs.

Geography
The layout of the Alternative Castle shares multiple graphics from the regular castle, yet often borrowing scrapped or unused graphics from earlier betas. The castle has many different areas that can appear depending on how personalized the copy is, which include modified versions of pre-existing rooms, like the Main Foyer, or new rooms, like the Library, Art Gallery, Upper Area, Garden and the Dungeon.

Because of how different the castles can be, many reports vary from one another.

Report 1
A video showcasing their version of the castle was uploaded by this YouTuber, and can be found here.

Main Foyer
The Main Foyer has three doors in the upper area, and two doors, which lead to the same room, in the lower area. In the upper area, the door on the left leads to a room with flowers, with the door on the right leading to a room with Cool Cool Mountain's beta painting. The middle door leads to the Second Floor.

Both doors in the lower area lead to the same circular room, with the eternal star statue in the middle and three doors. The doors on opposite sides of the room lead both lead to paintings of Bloobers and a modified rainy version of the Wing Cap Course. The painting of Bloobers implies that this player's copy has anomalous enemies as well. In between the two doors leading back to the Main Foyer is a staircase leading down into the basement.

Castle Courtyard
The Castle Courtyard is mostly unchanged, however, since the eternal star statue is now located indoors instead of outdoors, it has been replaced with a wishing well. A sign in the back of the yard mentions a course called Ghastly Graveyard, most likely replacing Big Boo's Haunt. Similar to Super Mario 64 DS, there is a small alcove above the door.

Basement
The main area of the Basement has three doors, leading to the Switch Dungeon, an unknown Bowser level, and corridors leading to more rooms. Inside one of the rooms is paintings for Shifting Sand Land, Lethal Lava Land, and an unknown course, with its painting featuring the Bowser in the Dark World skybox. Another room has a Thwomp trapped behind bars, as a warning for, "all those who disobey Bowser," and a painting for Motos Factory. The final room has a small pool of water, blocking off the entrance to a course called, "Colossal Cruiseship," and a metal door that can only be opened from the other side.

Second Floor
Despite the entrance to the Second Floor seemingly needing a key, the door can be opened without any issue. The Second Floor is a large empty room with pink walls, that has two paintings on opposite ends of the room, and four doors spread throughout. In the center of the room is a sun carpet, with a large hole in the ceiling. Two of the doors seem to lead to identical rooms, with the only difference being what painting is inside. One of the paintings seems to be a slightly modified version of the Lethal Lava Land painting, and the other appears to be Pokey Oasis. The other two doors lead to different rooms entirely, one being the Library. It also seems to strike a similar appearance to some renditions of the fourth floor.

Library
The final room explored is the Library. In the middle of the room is a moon carpet, surrounded by bookshelves. In the corners of the Library are two books, which, as a sign hints, can be entered to access two more courses.

The Hub
Upon entering the castle via the outside door, Mario lands in the Hub. The Hub consists of multiple branching hallways leading to paintings and a staircase leading to the Upper Area and the Basement. The graphic used for the walls in the Hub is the Third Floor's Starry Sky graphic. The paintings in the Hub require no stars to enter.

The Upper Area
At the top of the main staircase is the Upper Area, consisting of multiple bookshelves, in which Boos roam. Inbetween the bookshelves are red coins, which give Mario a secret star. An entrance to Bowser in the Sky lies at the end of the library of bookshelves, which is the only level in the Alternative Castle which is also in the regular castle. The Upper Area uses the beta castle wall textures.

The Garden
In one of the hallways in the Upper Hub is a pipe leading to The Garden. The Garden resembles the Castle Courtyard with some slight alterations: there are the unused dancing flowers scattered randomly, the Eternal Star uses a metallic texture and sometimes Goombas appear.

The Dungeon
The Dungeon is a small area accessed by ground-pounding into a specific spot of the carpet in The Hub, in which Mario will fall through a deep hole into The Dungeon's loading zone. The Dungeon is a labyrinth with multiple different rooms which, according to some reports, sometimes 'move around' and trap players in rooms. The walls use the Castle Basement wall texture.