The Castle Keeper

The Castle Keeper is an anomalous entity that only appears in the second floor of Princess Peach's Castle in certain personalized copies of Super Mario 64. It seems to be connected towards the "Please walk quietly in the hallway" sign in the floor, based on its behavior.

Description
The model of the anomaly is a plain white circle without any shading or textures, and has a low poly count. This is most likely a placeholder model that was meant to be replaced with a final one, if the Castle Keeper wasn't scrapped. As that's the case, it could be a sign that it was scrapped very early in development of Super Mario 64.

The entity is activated based on the speed of Mario. If Mario's fast enough while in the Castle Keeper's vision, it would follow Mario around like some enemies in the game slowly. Mario can go through the entity with no collision, and it can go through any object. The only audio that the Castle Keeper has would be its footsteps, which is the same audio of footsteps from certain bosses (Bowser, Big Bob-Omb, Whomp King, etc.).

Leaked Document
Around the early 2000's, a now defunct amateur gaming news website had made an article regarding an alleged leaked document about Super Mario 64. This leaked information was most likely about the Castle Keeper entity and how it was supposed to function, given the entity's behavior could connect towards the document. It's also where the entity's name came from due to said probability.

The Document
ULTRA 64 MARIO BROTHERS DESIGN DOCUMENT

BOSS ENTITY #3: The Castle Keeper

''The Castle Keeper would be put in the game as an optional boss that Mario would encounter in the second floor of Princess Peach's Castle. Activation of the boss would be based on the speed of Mario itself, and would require stealthy strategic maneuvers and planning to defeat it. A clue on the activation of the boss would be placed on a sign in the floor, in which it would tell the player to be quiet in the hallway. This would allow the player to know that something bad would happen if they make Mario run too fast.''

''The appearance of the boss would be a big, lumbering oni-like creature that has a stocky build. It would have green skin, an orange loincloth with black spots, two small eyes, two curled horns, and a giant spiked kanabō club it would use. The design of the Castle Keeper must invoke a sense of intimidation, but not too scary for children who play the game. This intimidating appearance would be used as a way to let players know what they would be against if they aren't quiet in the hallway.''

''The Castle Keeper would start off in the second floor of Princess Peach's Castle in the middle of the hallway room, sleeping on the job assigned by Koopa to squish Mario. The portly plumber must walk slowly around the Castle Keeper to not awaken it. If this is done for every single time Mario encounters the green ogre, he would be able to avoid the boss fight of it entirely. However, if Mario gets a bit too noisy and fast, the Castle Keeper would wake up and do its job to vanquish the poor plumber.''

''As the Castle Keeper being awakened would be a punishment for not following the instructions, it would be a difficult boss to fight against. The green creature would follow Mario around and slam his club to the ground to squish Mario. It would be slow in speed, but it makes it up with his long range due to his giant club. The damage of the club would take away half of Mario's health, instead of the normal three health points. The Castle Keeper would not be damaged by normal means, so avoiding it is the best choice. Mario can run away from the Castle Keeper and go on with his journey, and it would sleep again when Mario leaves the room.''

''There is a way to defeat the Castle Keeper and it involves unlocking the Metal Cap. Once unlocking the Metal Cap, Mario would use his new power-up to jump kick the green guardian. It would then fall over and would try to get up, which is where the plumber must ground pound him. The player must repeat this action two more times to defeat the Keeper.''

Whomp King Connections
The behavior of the Castle Keeper described in the document is similar to the Whomp King in Whomp's Fortress, albeit with more steps to defeat the former. Both are big bosses that would crush Mario and would be defeated with ground pounds. Since the Whomp King had not made an appearance in the known early beta builds of the game, it is possible that the Castle Keeper was an earlier version of the Whomp King or that some of the ideas of the Castle Keeper boss fight were reused for it.

Both the Castle Keeper and the Whomp King have connections towards mischevious creatures in Japanese lore. The Castle Keeper would be based off of the oni, while the Whomp King (and the Whomps) is based on the nurikabe. This could be a connection to the Castle Keeper being an earlier version of the Whomp King, or just pure coincidence.

"Please walk quietly in the hallway"
The infamous "Please walk quietly in the hallway" sign had been most likely used for the Castle Keeper boss fight, if the document's authentic. As the boss was scrapped in the final game, it is odd that the sign itself was still used. It was probably in the final game as a detail for the game; making it a sign for common courtesy while being in Princess Peach's Castle. However, people had theorized and thought that something else was intended for the sign, and believe that something (or someone) was in the castle.

The behavior of the anomalous entity that appears in certain personalized copies of Super Mario 64 also connects to the sign's details and is similar to the description of the Castle Keeper in the alleged leaked document. This could be a major connection towards the entity in the game and the Castle Keeper in the document. This is why the entity in the final game is named the Castle Keeper, as it's the most popular name given to it.