Keyhole

A Keyhole is a magical gateway in each world that leads to the Heart of that world. The Heartless invade worlds and search for their Keyholes (which, not surprisingly, appear as large keyholes). Upon discovering the Keyhole, the Heartless will enter it and devour the world's heart. After a world's heart is consumed, the world crumbles, and fades from existence. It is said that, if the Keyblade locks a world's keyhole, it can never be opened again.

Kingdom Hearts
The Heartless invaded 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 by going through a Corridor of Darkness (which opened during the islands destruction), while Riku ended up in Hollow Bastion. Kairi's body was found by Maleficent. In Traverse Town, Sora learned from Leon and Yuffie that the Heartless took over their world by order of Maleficent. Sora, along with Donald and Goofy, decide it is their duty to travel from world to world and seal the Keyholes using the Keyblade.

Maleficent and her gang of villains were in the midst of completing an artificial Keyhole in Hollow Bastion by obtaining the seven hearts of the Princesses of Heart. That Keyhole, once completed, would lead them into Kingdom Hearts, the heart of all kingdoms and source for unmeasurable power and knowledge. However, without Kairi's heart the Keyhole remained incomplete. Her heart was actually dwelling inside Sora's. Sora eventually released Kairi's heart, completing the Keyhole. Ansem, who possessed Riku's body, ventured into the Keyhole in order to get to Kingdom Hearts. Sora sealed the Keyhole and then followed Ansem to the End of the World, a place formed from the remains of worlds taken by the Heartless. Deep in the darkness of that place lay the Door to Darkness, with Kingdom Hearts lying beyond it. There, Sora battled Ansem. Ansem them opened the door so he could obtain the darkness inside Kingdom Hearts, but Sora called out the Light from inside the door and Ansem was destroyed. Sora, Riku and King Mickey sealed away Kingdom Hearts by locking the Door of Darkness on both sides. All of the destroyed worlds were restored, including Destiny Islands. It is presumed all the restored worlds had their Keyholes sealed automatically and hidden, so they could not be destroyed again.

Curiously, though several never-before-seen worlds appeared in Kingdom Hearts II, not one of them had their Keyholes sealed, mainly because after End of the World was destroyed, all worlds went back to what they used to be, and the newer worlds therefore had no reason to be sealed by a Keyblade.

Kingdom Hearts

 * Destiny Islands: The Keyhole takes the form of a wooden door with no knob in the Secret Place. It was opened by Riku.
 * Traverse Town: Traverse Town's Keyhole is located in the water fountain in the Second District. It only opens up when someone pulls the rope on the district's bell thrice, causing the water fountain to change face.
 * Wonderland: The Keyhole is appropriately hidden inside the mouth of the Doorknob.
 * Olympus Coliseum: The Keyhole was hidden under a stone block in the Lobby. It can only be accessed once Sora, Donald and Goofy use the yellow Trinity Mark to remove the stone block.
 * Deep Jungle: The Keyhole is hidden at the top of the waterfall rock face in the Cavern of Hearts, behind a flock of butterflies. The pathway into the waterfall rock is shown when Kerchak reveals it to them.
 * Agrabah: The Keyhole is in the final room of the Cave of Wonders, i.e. the Lamp Chamber. After stealing the lamp from Aladdin, Jafar's first wish was revealing it.
 * Atlantica: The Keyhole is hidden in Ariel's Grotto. It can only be accessed using a Crystal Trident or King Triton's trident. After defeating Ursula, Triton allowed Sora to lock the Keyhole using the trident to reveal it.
 * Halloween Town: The Keyhole was hidden under Oogie's Manor. After it collapsed, the moon reflected on the spot where the Keyhole was.
 * Neverland: Neverland's Keyhole is located on the clock face of Big Ben. It only reveals itself once the minute hand moves up to the strike of twelve.
 * 100 Acre Wood: The Keyhole is on the clasp that closes the book.
 * Hollow Bastion: The Keyhole of this world, also known as the Final Keyhole, has some controversies around it. On the Ansem Reports, Xehanort states that he found a strange door in the basement of the castle, the door had a large keyhole on it and it wasn't locked, so he opened it, seeing on the other side what appeared to be the world's heart. However, Maleficent was gathering the princesses to open the "Final Keyhole" on the top of the castle, where the in-game Keyhole was located. So it appears that the Keyhole found by Xehanort in the basement was the actual Keyhole to that world's heart, while the Keyhole Maleficent was working so hard to create was an artificial Keyhole to be used as a pathway to Kingdom Hearts. This could probably explain why Hollow Bastion wasn't destroyed along with the rest of that world.

Trivia

 * In Kingdom Hearts II, when Sora first hears from Merlin that he has found Winnie the Pooh's story book, if one looks closely as Sora enters, they can see the clasp on the book clearly shows the sealed Keyhole, even though that was an optional Keyhole in the first game.
 * The Deep Jungle and 100 Acre Wood Keyholes are the only Keyholes in Kingdom Hearts that remain visible after being sealed. In the 100 Acre Wood it remains visible on the clasp, and in the Deep Jungle it remains visible as a keyhole-shaped clearing in the vines.
 * If the Keyhole to Hollow Bastion was already discovered in the basement, but never sealed, it is questionable why the Heartless never destroyed the world. It is possible Xehanort still had a use for Hollow Bastion and therefore commanded the Heartless to not destroy it.

Serrure