Princess Peach's Castle/The Throne Room

The Throne Room is an anomalous room that has several manifestations, making it hard to define as a single area. It has been reported to appear in Princess Peach's Castle, Bob-omb Battlefield, Whomp's Fortress (and Flying Fortress as well as Whomp's Fortress Interior by extension), and, most debatably, the Eternal Fort. It is also associated with an anomalous entity referred to as The King, appearing in most manifestations of the Throne Room.

Princess Peach's Castle
The Princess Peach's Castle manifestation of the Throne Room is the most common appearance of the Throne Room. It usually appears on the Third Floor at the top of the endless staircase, or anomalous floors such as the 4th Floor. It usually simply functions as a landmark, simply being a room that can be explored, occasionally containing a level or two, can branch off into other areas such as the Castle Balcony, or contain some Castle's Secret Stars.

According to some, after the ending (unless an Alternate Ending occurs), Princess Peach can be found in the Throne Room, and after some dialogue, can unlock something, nothing at all, or give a 121st Star. In these instances, The King appears to not manifest unless something that occurs that causes an exception.

Bob-omb Battlefield & Whomp's Fortress
The manifestations of the Throne Room in Bob-omb Battlefield & Whomp's Fortress are fairly similar, so they will be grouped together. The Throne Room usually appears at the top of the levels, in a castle or similar structure. They usually function as the arena for the boss fights against Big Bob-omb and King Whomp respectively, unless they are replaced with another boss that is still considered "royalty". The King has never been reported in these manifestations of the Throne Room.

The Eternal Fort
This manifestation's authenticity is dubious as best, as almost all reports are unconfirmed, as well as the Eternal Fort's nature making it almost impossible to know how many times the Throne Room can supposedly manifest in this level. The Throne Room can reportedly just function as a landmark, or function as the "exit" of the level. The King has been reported in every report of the Throne Room appearing in the Eternal Fort, and is much more vague than normal in these reports. They are also a much more prominent figure in these reports, despite their mysterious manifestations.

The King
The King is an entity associated with the Throne Room that has only been reported in the Throne Room. Their appearance has almost never been described by any reporters for varying reasons, with some saying it was invisible or blocked out by some form of censoring, usually being a black box. The King has semi-consistently been reported as an important character, being referenced by several NPCs in copies that have the Throne Room as well as the King.

The King, when it's appearance is described, most commonly in the Princess Peach's Castle manifestation, is most commonly represented as a large Toad with a different color, sometimes appearing similar to Toadsworth who appears later in the Super Mario Bros. series, implying Coincidental Character Creation or that this was the original origin of the character. In this manifestation, they most commonly simply have dialogue about hoping the player will save Princess Peach.

However, the King's nature in the dubious Eternal Fort manifestation of the Throne Room is much different from it's description in Princess Peach's Castle. They are never properly described, except for some references to their dialogue, mainly being that they are surprised the player got through the Fort and "wishes to talk to them". Not much else has been confirmed about this interpretation of the King.

BlackJoystick's Documentation
BlackJoystick is one of the few to have a confirmed report of the Throne Room being in the Eternal Fort. They access the Throne Room after exploring seemingly endless halls that appear to be in the Eternal Fort, with a carpet leading up to the only double door seen in the area.

The Throne Room is very briefly explored, with it leading to a balcony where 4 towers that are the colors of the ! boxes seen in the standard copy, as well as a Toad that states:

"I must admit, you're quite the special one for getting so far and deep into this sacred place. The King is impressed with your skills. He's waiting for you at his personal chamber up ahead. Don't keep him waiting, you won't like to see him when he's mad, trust me."

BlackJoystick then goes to the King's room, where they meet the King, who states upon them entering:

"Why, it seems that you have finally arrived. And just in time too... Why don't you come closer? I'd like to have a talk with you."

The King's appearance is censored by a black box, as well as some of their dialogue, making it hard to decipher what they're saying. Eventually, the King appears to warp BlackJoystick somewhere as the video ends, potentially due to Star Road which supposedly appears after the Eternal Fort according to BlackJoystick.