Ursula's Lair

Ursula's fortress is the home of the evil seawitch, Ursula. After she was exiled from King Triton's palace, and consequently Atlantica, Ursula made herself at home in this location. The lair itself is a colossal skeleton of a beast known as the Serpentine. The gaping mouth serves as the main entrance, with an entry corridor, containing Ursula's garden of polyps, that leads to the main hall containing Ursula's cauldron, a vanity wardrobe, a cupboard for her ingredients, and a large conch that acts as Ursula's perch. Behind the conch is a small entryway to the back of Ursula's lair. And there are also a few holes in the roof that lead to the outside of the lair.

The lair would later appear in the television series.

Deleted Scenes
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 along with her nautilus shattering.

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.

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.

The lair never appeared in Kingdom Hearts 2.

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 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.