World Cards

World cards are a type of map card in Kingdom Hearts: Chain of Memories which can transform floors of Castle Oblivion into entire worlds made out of Sora or Riku's memories. These cards are usually created by members of the Organization, but DiZ and Naminé are both able to make cards for Riku, and one appears on its own to King Mickey. Naminé is even able to fashion these cards into trinkets, as she does for the Riku Replica.

The world cards are not all available immediately; rather, new cards become available once all of the cards currently available have been used.

World cards reappear in Kingdom Hearts coded, where they allow Data-Sora to change the rooms of the digital Castle Oblivion into representations of the worlds he has visited.

Kingdom Hearts: Chain of Memories
Upon entering Castle Oblivion, Sora, Donald, and Goofy encounter the mysterious figure, who samples Sora's memories and gives him a card for Traverse Town. They encounter the figure again once they reach the 1F Exit Hall, but when Axel appears, the figure gives him a number of cards and leaves him to test Sora. After defeating Axel, Sora receives the cards for Agrabah, Olympus Coliseum, Wonderland, Monstro and Halloween Town. As Axel and Larxene observe the group reach the sixth floor, Larxene declares it to be her turn to face Sora, and Axel gives her more cards. After she is defeated in the 6F Exit Hall, she tauntingly gives Sora the cards for Atlantica, Neverland, Hollow Bastion, and the 100 Acre Wood.

In the meantime, Riku awakens in a strange place, and a voice offers a way out by presenting him with the card to Hollow Bastion. When Riku takes hold of it, the card takes him to the B12F Entrance Hall. After leaving the floor, Riku is confronted by and fights Ansem, Seeker of Darkness, who gives him cards for Traverse Town, Agrabah, Monstro, and Neverland, as well as providing Riku with the use of Dark Mode. After Riku reaches the B7F Entrance Hall, Ansem appears again, and he gives Riku the cards to Olympus Coliseum, Wonderland, Halloween Town and Atlantica, challenging him to continue resisting the darkness.

When Vexen brings the Riku Replica to the upper floors, Axel gives him the Destiny Islands card, which Naminé uses to change the replica's memories to think he is the real Riku. She also transforms the card into the form of a good luck charm.

Vexen gives Sora the Twilight Town card when he is defeated in the 10F Exit Hall, saying he made it from the memories locked in the other side of Sora's heart.

After fighting the Riku Replica for the third time, in the 11F Exit Hall, the replica drops his good luck charm, which transforms back into the Destiny Islands card in Sora's hand.

When Riku reaches the B3F Entrance Hall, Zexion gives him the Destiny Islands card. Mickey Mouse finally catches up to Riku in the B2F Entrance Hall, and he gives him the Twilight Town card, saying it led him from the Realm of Darkness to where Riku was. At the Exit Hall, DiZ gives Riku a Castle Obvlivion card to draw out Ansem from within his heart.

Kingdom Heart coded
Upon reaching Castle Oblivion, Data-Sora encounters a hooded figure who gives him the Destiny Islands card and tells him to use it to find the "truth that sleeps". After Sora clears the room, the figure taunts him about his lost memories and gives him the cards for Traverse Town, Wonderland, Olympus Coliseum, Agrabah and Hollow Bastion.

After Data-Sora has used all of the cards, the figure confronts him again and engages him in battle. After Sora defeats him and accepts the burden of hurt into his heart, the figure gives him the Castle Oblivion card, which leads him to Data-Naminé's room.

Trivia

 * Although Sora, Donald, and Goofy visited the world Deep Jungle in Kingdom Hearts, it was not included in Kingdom Hearts: Chain of Memories as the estate of Edgar Rice Burroughs did not allow Disney and Square Enix to reuse the story. It appeared in early treatments for the game, though.
 * Though Castle Oblivion as a World Card is not obtained by Sora, and does not appear in his "World Cards" menu in Kingdom Hearts: Chain of Memories, it does appear in his "World Cards" menu in Kingdom Hearts Re:Chain of Memories, and even uses the image of Riku's Castle Oblivion card as its depiction.
 * During cutscenes in Kingdom Hearts Re:Chain of Memories, the Destiny Islands card shows the Kingdom Hearts II design rather than the one used in Kingdom Hearts: Chain of Memories, however, once you actually receive the card, the design from the original game is shown to be on the card instead. The reason for this is unknown.
 * These cards may be the mechanism by which the Organization takes Sora, Donald, and Goofy's memories, and may have actually been created by Naminé. Except for in Traverse Town, Sora, Goofy, and Donald seem to have lost all their memories of each world they visit before they enter them, and the cards are said to be made by using their memories. However, they were able to at least recognize that they've been to each world, as they comment in Twilight Town that they don't recognize it.

Notes and references
Cartes Mondes