Princess Peach's Castle/Second Floor

The second floor of Princess Peach's Castle is an area accessed in the latter half of Super Mario 64. After collecting 31 stars and defeating Bowser in Bowser in the Fire Sea, Mario can use the key to access the second floor of the castle.

Geography
The second floor can be found past the double door located up the stairs in the main foyer. These doors lead to a spiral staircase that leads up to the second floor itself.

The second floor is a donut-shaped room with blue sky-like walls. It has many paintings which cannot be entered, except for a few: a painting of a Skeeter leading to Wet-Dry World, and a painting of Tall, Tall Mountain. There's a staircase on this floor that leads up to the third floor of the castle.

There is a door that leads to a triplet of identical paintings, all with different sizes. An example of perspective trickery has the room get progressively smaller or progressively larger, approaching the smaller and larger paintings. This leads to Tiny-Huge Island.

Mirror Room
There is another door, surrounded with brown bricks, which leads into the infamous mirror room. This room is characterized by its large mirror, which mirrors Mario and the Lakitu filming him. There are a series of paintings in the room, although there is a bland wall that appears as a snowman painting on the other side of the mirror. This empty wall leads to Snowman's Land.

Phenomena
'Please walk quietly in the hallway'

Although isn't an actual phenomenon, the textbox's real meaning is a mystery, because the place it is in is not a corridor and walking quietly will have no effect, and neither does running or making noise.

In the first video of Greenio's odd findings, it is theorized that the textbox would have been used in the basement, specifically, to not wake up Wario.