Princess Peach's Castle/Castle Courtyard

The Castle Courtyard is accessed from the First Floor of Princess Peach's Castle. It contains the Eternal Star Statue. Once 12 stars have been collected, Big Boo's Haunt can be entered here. It is a small area behind the castle, being a circular area with brown brick walls surrounding the premises and the entry door, gray flooring, and grass with several trees unique to the area. In the center, a fountain with the Eternal Star Statue is present. Green hills can be seen surrounding the walls. There are also signs scattered around the area. Interestingly, this is the only area of the castle that has enemies in it, with Boos appearing here once the player has collected enough stars.

L is Real 2401
Main Article: Luigi

This is the most popular anomaly associated with Super Mario 64. If one attempts to read the text on the plaque of the Eternal Star Statue, one may interpret it as "L is Real 2401". This has led to many believing Luigi is in the game, and 2401 is somehow associated with them. Alternatively, it can be read as "Eternal Star", which has led to an entire different variety of rumors.

Alcove
An alcove with a 50-star door may occasionally be added above the door, requiring jumping from a tree to reach. It usually leads to the second floor, sometimes to a new room which has been reported as one way of reaching the other side of the Mirror Room.

Replacements
Main Article: Castle Garden, Goomba Courtyard, Castle Fields

Sometimes, the Castle Courtyard can be replaced with anomalous areas. The most common of these is the Castle Garden, a larger, more open space with similar geography minus the fountain and a unique Dark Room. There is also the Goomba Courtyard, an anomalous version of the courtyard full of Goombas, and the Castle Fields, another large open space without any walls made up of various hills.

Hollow Horror Halls
Main Article: Hollow Horror Halls

Hollow Horror Halls is an anomalous level that can sometimes be found in the Castle Courtyard. How is inconsistent, but some reports say it can be found in place of Big Boo's Haunt, in a second cage that can spawn, or by entering a door somewhere in the courtyard. The level itself is a large maze with a similar theme as Big Boo's Haunt that is considered rather difficult by those who have played it.

Bowser in the Castle Courtyard
Main Article: Bowser in the Castle Courtyard

Bowser in the Castle Courtyard is the name given to an anomaly where Bowser can spawn in the Castle Courtyard and fight the player after they beat the main game. This usually is a much harder fight as there are no bombs in the courtyard. This can result in unlocking Luigi, likely due to the Eternal Star Statue's connection to the Luigi rumors. Why this occurs is not yet understood, and may have been part of a secret ending.

Beneath the Statue
In some copies of the game, it is possible to move the Eternal Star statue in the middle of the courtyard and uncover a secret area beneath.

Moving the statue consistently requires the Metal Cap. The means of obtaining it however vary quite drastically: while it has in a few instances simply been added to the courtyard it often requires relatively major personalization to manifest, such as expansion of the courtyard with a hedge maze wherein the cap is found, or a locked door at the other side.

One player even reported needing to pass an "obstacle course" on the second floor to bring the Metal Cap to the courtyard in time; this obstacle course was notably difficult compared to unpersonalized stages and required several tries to complete.

The contents of the area vary too: it usually manifests as a small room with a Castle's Secret Star and an one-way door into the basement, but has also been reported to contain a secret level of some kind. Notably, it is both rumored to be one of the locations for the fabled Yellow Switch level and a means of unlocking Luigi.