Door to Darkness

The Door is an object from the Kingdom Hearts games.

A trademark of the series, the Door is a tall, pink door first seen in Kingdom Hearts in the Awakening. Its appearance in a world marks the involved world's connection to other worlds (i.e. the Destiny Islands). The Door is used by the Heartless to travel from one world to another. It is referred to by Ansem multiple times ("The Door is open.").

The true location of the Door is within a cave in End of the World beneath Bald Mountain. The name of the cave is Final Rest. It is also called the DTD.

Kingdom Hearts II
In Kingdom Hearts II, the Door To Darkness is connected to Space Paranoids. Tron needs a password to access the "DTD", and when Sora, Donald Duck, and Goofy return to Radiant Garden, King Mickey reveals that the "DTD" refers to the Door to Darkness. And since the seven Princesses of Heart were required to open the Door to Darkness, the names of the Princesses were the password.