Eternal Fort

The Eternal Fort is an elusive and mysterious course from Super Mario 64 spread by rumour and word of mouth. The existence of this area in the final game is heavily debated, and the few that insist that they've stumbled across it either refuse to divulge details about the level itself or go dead silent after saying so.

Information
Information on the Eternal Fort is scarce, with no coherent image or depiction of the level itself. What is known is that the Eternal Fort was intended to be the ultimate and final level in Super Mario 64, but was removed before the final build for unknown reasons. It's also unknown if this level was intended to be in place of Bowser in the Sky, or as a bonus level after beating the game.

A video surfaced online claiming to be the extracted music associated with the Eternal Fort, but viewing of this video could potentially be dangerous as hinted at by the description; Nintendo appears to be keeping this level under wraps for an unknown reason.

There are very minimal credible sources that refer to Eternal Fort, and even the ones that do are often very vague, in passing or go radio silent after referring to it. The ones that do speak up usually only have years after, and either denounce the game entirely or appear utterly distressed over whatever they came across.

Speculation
Given by the words that Nintendissident and the King of the Internet have said about the Eternal Fort, it appears that Nintendo is intentionally keeping the existence of the level under wraps and down low. The reason why is unknown, but the two users discuss the idea of a "final truth" hidden within the level.

The Eternal Fort's existence is tied to the Personalisation A.I., where many believe people will only find it if they find out everything there is to know about Super Mario 64. On an alternative and more innocent note, the level could've proven too difficult and Nintendo removed it due to its difficulty. Therefore, the A.I. only summons the level for experienced players who have proven themselves worthy of playing the level.

Connections to Other Areas
Fortresses are a common level theme in the Super Mario series, and Super Mario 64 is no exception. The entirety of the game's hub world is situated in Princess Peach's Castle, which could be seen as a mirror to the Eternal Fort in a way. The Eternal Fort may also be tied to the similarly named Eternal Star Statue as seen within the Castle Courtyard. Not only do the two share similar names, but the King of the Internet refers to the infamous L is Real legend in his comment. With that, it can be assumed the Eternal Star Statue is related to or a remnant of the Eternal Fort, or that Luigi is associated with the Eternal Fort.

Alternatively, the Eternal Fort could be seen as the Mushroom Castle's opposite: Bowser's Castle. Interestingly, Super Mario 64 was the first game to feature Bowser as a main antagonist yet not have his castle present in the game at all. Instead, Bowser has the three levels dedicated to him on each level of the castle. It's possible that the Eternal Fort is Super Mario 64's iteration of Bowser's Castle judging by how it was originally intended to be the final stage of the game. Some unverifable sources refer to the level as "Bowser in the Eternal Fort". Interestingly, the second floor of Peach's Castle is the only floor to not feature any Bowser level. It's possible that the Bowser level intended for this floor was Bowser in the Sky, and the Eternal Fort was to take its place on the third floor. On the other hand, it could've also been the Bowser level of the fourth floor.

An interesting theory is that part of the Eternal Fort is seen within Tower of the Wing Cap, which headlines four large towers situtated around a larger flat-roofed tower in the middle of the sky. It's possible that the tower seen here are a part of the Eternal Fort, as these towers could be just a segment of a much larger structured as suggested by the fading texture at the base of these towers, meant to simulate these towers going down into the clouds.