In the entrance hall of Castle Oblivion, Marluxia gives Sora a card for Traverse Town, saying that he sampled Sora's memories to make it. After Sora enters the memory projection of the town, Marluxia introduces him to the mechanics of using cards in battle, and he explains that everything in the castle is determined by cards, from fighting to progressing through the rooms of the castle. In the Room of Beginnings, Sora encounters Leon who explains more about using cards in battle, including card values and stocking cards. Sora obtains additional world cards from Axel and Larxene after he fights each of them for the first time. In order to test Sora and the Riku Replica, Vexen, Axel, and Larxene have Naminé transform the Destiny Islands world card into the Replica's lucky charm, which remakes his heart so that he believes he is the true Riku. After meeting with Sora as planned, Vexen seizes the opportunity to prevent Marluxia's coup by sampling Roxas's memories via Sora and crafting from them the world card for Twilight Town, which he gives to Sora while inviting him to question whether his faith in Naminé is true or an illusion. Afterward, when Sora defeats the Riku Replica in the 11F Exit Hall, the Replica drops the charm and it returns to its card state in Sora's hand.
Meanwhile, Riku rests in the Realm of Darkness, where he is presented with a card for Hollow Bastion by DiZ, who communicates as an unnamed voice. Choosing to take the card, Riku appears in the twelfth basement of the castle and makes his way through it, and is encountered by the voice on the other side. DiZ, disguising himself as Ansem, introduces Riku to card duels and gives him a set of world cards. Riku faces this Ansem again in the B8F Exit Hall, and is given another set of cards. Riku encounters Zexion at the third basement, and the Organization member attempts to break Riku's spirit and make him believe that he is a villain that must be slain by Sora by giving him a card for Destiny Islands. After clearing the floor, he finally meets up with Mickey Mouse in the B2F Entrance Hall, and Mickey gives Riku the card for Twilight Town that he says led him to the warrior. Within Twilight Town, DiZ reappears as Ansem, but the more confident Riku sees through the masquerade and so DiZ advises him to speak with Naminé. After that conversation, Riku decides to confront his darkness once and for all, so DiZ gives him the Castle Oblivion card, which Riku uses to draw out the true Ansem for a final battle.
There are three types of cards that appear in Castle Oblivion. They are World Cards, Map cards, and Battle cards.
World cards open the entrance to a floor of the castle, changing it to reflect the memories contained in the card. Inside, one will encounter characters they have met in the card's world and experience events that reflect their memories of that world.
Map cards are used outside of battle to open the many rooms on the floors of Castle Oblivion. They affect the size of the rooms and the contents, including enemies, chests, and shops, and they can affect the flow of battles within the rooms. Map cards can be used to open closed rooms, or to change the contents of rooms previously explored.
Battle cards are used when fighting enemies, replacing most abilities from Kingdom Hearts, including weapon skills, magic, summons, items and passive abilities. Depending on the card, they have different value points which determines the "strength" of the card and it costs Sora Card Points (CP) to add to a Deck once added. Riku however, cannot customize his battle deck, and he uses a preset battle deck instead.
Kingdom Hearts coded / Re:coded
World cards are once again utilized in the game's final world, Castle Oblivion, as a means of accessing higher floors. Unlike in Chain of Memories and its remake, the effects of World Cards do not completely transform the appearance of individual floors to reflect the world on the card, but rather appear as two-dimensional illusions on the walls.