Unused Level 1

Unused Level 1, filename 1MARIO.bin, is an unused level from Super Mario 64 that was featured in prerelease footage of the game (most notably in one of the B-Roll clips for it). The ID for this level is in the level select debug feature in the game, ableit without any content found. The entire level was found via the source code for the iQue version of Super Mario 64 from the July 25th 2020 leak. A duplicate version of the level, dubbed Unused Level 99, was also found in the leak. The duplicate version contains some differences to the level.

Contents
Unused Level 1/99 takes place in the middle of castle-like area, not unlike the small fortress found in Whomp's Fortress. There are two towers found in the level; one that looks like one of the tower's from Princess Peach's Castle's exterior, and the other that's in the topmost area with a texture on top that looks like one of the beta textures of the Thwomp enemy. Some parts of the area contain slopes and steps for Mario to reach other parts of the level, while others seem inaccessible without the use of cheats or debugging.

An interesting part of the level itself would be that there's functional water in the lower areas of it, albeit with either no water texture on top of the water to show that it contains water or some parts of the underwater area containing it.

In Unused Level 1, there seems to be no other entities found within the area besides Mario himself. There are no Power Stars to collect in this version of the level, so this could be an earlier version. However, Unused Level 99 contains some objects in the game, including blue poles, some coins, a Power Star, and strangely a fish not seen in the underwater areas of the level. It is also said that there also contains beta versions of the Power Star and the Bob-Omb enemy in the code of "Level 99".

Theories
One theory about Unused Level 1/99 would be that the level itself was an early version of Wet-Dry World, considering that it used similar texturing to the underwater city area found in the level. Since the object data of Unused Level 1/99 seems to be rudimentary at best, it is uncertain that this could be the case.

The level data for Level 99 has a star at the top of the level that's notably just out of reach for Mario to grab, even with a triple jump. It's possible this might mean that Mario was able to jump higher early (or the gravity was lower) in development, which would have allowed him to properly collect this star. In the final game, stars are usually placed so that Mario does not have to do a triple jump.

Another similar set of theories would be that it had something to do with Whomp's Fortress, with a similar theme of castles being used for both areas. One of these theories state that this was a very early version of Whomp's Fortress stored in the code for the game that was later supposed to be used as another level or used as the inside of the tower after beating the Whomp King. This could be similar to how Cavern of the Metal Cap was used in Hazy Maze Cave. As such, it could also be possible that the Yellow Cap Switch was meant to be used here. Some of the textures found in Unused Level 1/99 are either similar to or the same ones used in the tower of the Spaceworld version of Whomp's Fortress, further strengthening this theory.

Trivia

 * Unused Level 99's Power Star is dubbed the "true 121st Star" by some parts of the community, as a similar star discovered earlier known as the Toad's Tool 64 Star seems to be intangible and only appears in the emulator Toad's Tool 64.