The Princess' Secret Slide

The Princess' Secret Slide is a hidden level that can be found in Super Mario 64. It is found by entering a 1 star door in the First Floor of Princess Peach's Castle and jumping into a stained glass window in the right wall of the room. There are 2 stars in this level, both considered Castle Secret stars.

Geography
The Princess' Secret Slide starts in a room with red carpet and grey brick walls and a roof. There is a sign and a path that leads to the slide. The slide is a long orange slide that twists downwards to a second room similar to the first one, this time with a platform with a yellow ! box that when broken, reveals a star.

Star 1
The first star in The Princess' Secret Slide is collected simply by reaching the end of the level.

Star 2
The second star in The Princess' Secret Slide is collected by reaching the end of the level in under 21 seconds.

Secret Slide Dimensional Rift
The slide is located in a black void, though it is accessed via stained glass, leading some to believe it is another dimension.

Yellow Cap Switch
Found in the files is a Yellow Cap Switch referenced to appear in The Princess' Secret Slide that would have unlocked Koopa shells.

Springboard
An unused Springboard object is referenced to appear in The Princess' Secret Slide, similarly to the Yellow Cap Switch.