The Door is an object from the Kingdom Hearts games.
A trademark of the series, the Door is a tall, white door first seen in Kingdom Hearts in the End of the World. Beyond the door is the Realm of Darkness where Kingdom Hearts, the heart of all worlds, resides. It is referred to by Ansem multiple times ("The Door is open.").
After the battle with the World of Chaos, Xehanort's Heartless opens the door and asks that Kingdom Hearts fill him with the power of darkness. But to his horror, Kingdom Hearts is revealed to be light and it destroys him.
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.