Ursula's Lair

Ursula's fortress is the home of the evil seawitch, Ursula. After she was exiled from Atlantica by King Triton.

The lair would later appear in the television series.

Origins
The Little Mermaid comic, Serpen Teen, the creature which made the skeleton was once known as, the Serpentine. The creature was used by a kingdom of eel-human hybrids known as the moray as a form of gladitorial entertainment. They would send unsuspecting creatures to the maze where the creature lurked and watch as it fed upon them.

During the events of the comic, Ariel had managed to elude the creature to the point where it was sent into a bloodthirsty rage, which forced the moray to kill it. Then Ursula came along and offered the moray a chance to steal King Triton's trident if they gave her the creature's skeleton.

From that point forth Ursula used it as her lair.

The Layout
The lair is a skeleton of a gigantic creature long since dead. It's maw is the main entrance, but due to the decay, there are many ways to enter and exit the place.

Past the main entrance is a single corridor that leads to the main hall. In this corridor is Ursula's garden of polyps writing about in agony. Eventually the polyps give way to the entrance leading to the main hall.

The main hall is the center of most activity in the lair. In the middle of the room is a large cauldron, kept inside a sort of pit in the ground. There are pink plants hanging from everywhere. Then to the far right of the entrance, is Ursula's mirror and vanity, where she pampers herself up with flower lipstick, and her other cosmetics, all kept organized on the individual arms of the mirror frame.

Ursula's throne is directly ahead of the main entrance, hanging from the cieling and blocking the entrance to the back corridor. Ursula sits in this area and possibly uses it as a bed at nights. She keeps a clam filled with live shrimp inside, so that she may snack on them whenever she desires to.

And to the left of the entrance, (hanging from the wall) is Ursula's potion cabinet, where she has all sorts of ingredients tucked away for her spells. Most of these are made of various sea creatures, whether alive or dead.

Kingdom Hearts
In Kingdom Hearts, The lair was referred to as the Leviathan. Nothing was changed about the decor of the lair, except for it's missing back corridor. What's interesting to note about the Leviathon is that Ursula's garden is never transformed back into merfolk after Ursula is killed.

Also of note is that the way to get to the lair has changed. Originally, Ariel had to cross volcanic open sea to get to the lair, but in the game, Sora must go through the ship graveyard to reach the entrance.

The lair never appeared in Kingdom Hearts II.

Trivia

 * In the script for the Little Mermaid, Ursula's Lair was named, "The Whale Skeleton" despite the appearance of the creature in the final cut having no resemblance to a whale.
 * In an original draft for the Little Mermaid, Ursula's fortress was originally supposed to be destroyed as soon as she was killed by  Prince Eric.
 * In the video game for the NES and Game Boy and known as "Ursula's Castle" as the fifth and final level before the final battle, it is a much different layout and has stories where Ariel encounters Ursula (the boss) on the highest story.