The Guard Tower

The Guard Tower is a mysterious, very lowly documented phenomena within Super Mario 64. Players who have experienced it recall a large tower existing west of the castle that either disappeared at one point in their playthrough before being able to access it or lost access to the game and never saw it again. For that reason there are many theories guessing at lies within the tower.

Location/Appearance
Players recall the tower being tall, skinny, and having a door on the top with no platform leading to it. It was apparently located behind the hills and west of the waterfall.

As most players who witnessed the tower were usually very young at their time of playing, they lacked the experience or the knowledge to try and navigate up to the tower.

Disappearance
A group of players recall that the tower no longer appeared on the castle grounds after they progressed into the game a certain amount, tried to tell their friends about the tower, or tried researching how to access it online.

Other accounts simply say they stopped playing or they lost their copy of the game, never thinking too much about it until recently.

According to these accounts, the tower seems to remove itself after players have tried to bring it into attention of other people or if players simply didn't care about it. This also brings out the possibility that there might be players out there that had a guard tower in their castle grounds but never bothered noticing it, making it remove itself.

Theories/Speculation
Since no players that witnessed the tower were able to enter it, there are a number or theories pertaining to the contents of the tower or why it exists in the first place.

Yoshi Room Theory
The Yoshi Room Theory suggests that the tower would be the new meeting place for Yoshi at the end of the game and would contain the Wing Cap Box and 1ups. These elements would be missing from the top of the castle as well.

Post-Game Boss Rush Theory
The Boss Rush Theory suggests that the tower was the entrance to a boss rush to reward players who have beaten the game with a new challenge akin to other post game levels in 3D Mario Games.

Secret Star Theory
The Secret Star Theory suggests that the tower was the entrance to a Secret Star bonus stage. The contents and theming of the level were unexplained, though.

Motivational Tactic Theory
The Motivational Tactic Theory suggests that the tower is generated as a way to motivate players to progress with the game if the player hasn't made any progress after a specific amount of time. this tries to give an explanation for why the tower seems to pop in and out of existence.