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, not surprisingly, appear as 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 is 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 learned 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. During their journey, Sora, Donald, and Goofy go to Agrabah where Genie claims he has heard of the Keyholes before.

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 sealed away Kingdom Hearts by shutting the Door of Darkness on both sides sealing it as well. All of the destroyed worlds are restored, including Destiny Islands. It is presumed all the restored worlds had their Keyhole sealed automatically and hidden, so they could not be destroyed again.

In Castle Oblivion, featured in Kingdom Hearts: 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 Riku.
 * 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 ironically is shaped like a keyhole.
 * Deep Jungle: The Keyhole is hidden at the top of the waterfall rock face in a small cave, behind a flock of butterflies.
 * Olympus Coliseum: The Keyhole is kept hidden until Sora gains the ability to access the yellow Trinity Mark. 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. After stealing the lamp from Aladdin, Jafar's first wish was revealing it.
 * 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 reflected on the spot where the Keyhole is.
 * Neverland: Neverland's Keyhole is located on the clock face of Big Ben.
 * Hollow Bastion: Hollow Bastion's Keyhole is located in the Dark Depths, beyond the Grand Hall. 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.
 * 100 Acre Wood: The Keyhole is on the clasp that closes the book.
 * Castle Oblivion: The Keyhole is on the top floor after Sora defeats Marluxia. It should be noted, however, that this has not been confirmed to be a true Keyhole, and may simply represent the locking of the door.