Keyhole

Keyholes are a magical presence in each world that protects the Heart of the World. The Heartless invade worlds and search for their Keyholes (which actually are large keyholes). Upon discovering the Keyholes, the Heartless enter them and devour the world hearts, destroying the worlds and sending them to the End of the World, any survivors of the worlds arriving in Traverse Town usually. Sora, wielder of the Keyblade visits several worlds which the Heartless have invaded and seals the Keyholes.

Kingdom Hearts
The Heartless invade Destiny Islands, the home of Sora, Riku and Kairi. Not only did they attack the islands in search of the Keyhole, but also to find the Keyblade Master and one of the Seven Princesses of Heart, who are Sora and Kairi. The islands were eventually destroyed when the Heartless attacked Kairi and managed to get into the Keyhole (located in the Secret Cave). Sora ended up in Traverse Town, while Riku ended up in Hollow Bastion. Kairi's body was found by Captain Hook. In Traverse Town, Sora learnt from Leon and Yuffie that the Heartless destroyed his world in search for the Keyblade. Sora, along with Donald and Goofy, decide it is their duty to travel from world to world and seal the Keyholes using the Keyblade. They later learn that even Traverse Town has a Keyhole.

Maleficent and her gang of villains are completing Hollow Bastion's Keyhole by obtaining the seven hearts of the Princesses of Hearts. However, Kairi's heart is missing and is actually dwelling inside Sora's. Sora eventually releases Kairi's heart and the other hearts too, completing the Keyhole but he becomes a Heartless briefly. Ansem who is possessing Riku's body, ventures into the world's Keyhole to the End of the World. Sora seals the Keyhole and then has to use an alternate route the reach the End of the World. There he battles Ansem. Ansem is destroyed by Kingdom Hearts. Sora, Riku and King Mickey seal Kingdom Hearts by shutting the Door of Darkness on both sides. All of the destroyed worlds are restored, including Destiny Islands. It is presumed all the destroyed worlds had their Keyhole sealed automatically or hidden, so they could not be destroyed again.

In Castle Oblivion, featured in Chain of Memories, Sora defeats Marluxia of Organization XIII and then seals the castle's Keyhole on the top floor.

World Keyhole Locations

 * Destiny Islands: The Keyhole takes the form of a door in the Secret Cave. It was opened by either the Heartless or Kairi.
 * Traverse Town: Traverse Town's Keyhole is located in the water fountain in the Second District.
 * Wonderland: The Keyhole is appropriately hidden inside the mouth of the Doorknob, which is shaped like a keyhole.
 * Deep Jungle: The Keyhole is hidden at the top of the waterfall rockface in a small cave. The Keyhole is hidden behind a flock of butterflies.
 * Olympus Coliseum: The Keyhole is kept hidden until Sora gains the ability to access all of the Trinity Marks. It is hidden under a stone block in the coliseum's lobby.
 * Agrabah: The Keyhole is in the final room of the Cave of Wonders.
 * Atlantica: The Keyhole is hidden in Ariel's Grotto. Sora, Ariel and the others found its primary piece in a shipwreck but it was destroyed by King Triton. After defeating Ursula, Triton allowed Sora to seal the Keyhole using the Trident.
 * Halloween Town: The Keyhole is hidden under the house of Oogie Boogie. After it collapsed, the moon relfected on the spot where the Keyhole is.
 * Neverland: Neverland's Keyhole is located on the clockface of Big Ben.
 * Hollow Bastion: Hollow Bastion's Keyhole is located in the central room of the castle. Maleficent needed the seven Princesses of Heart to complete the Keyhole and gain true power of darkness. It was completed when Sora released Kairi's heart from his. It was finally sealed by Sora on his return trip to Hollow Bastion.