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. Naminé uses her powers to transform a card of the Destiny Islands into a lucky charm that is carried by the Riku Replica. After Sora defeats him in the 11F Exit Hall, the Replica drops the charm, which turns back into a card in Sora's hand.
Meanwhile, Riku appears in the twelfth basement of the castle, where he is presented with a card for Hollow Bastion. After clearing the floor, he is confronted by Ansem, who introduces him to card duels and gives him a set of world cards. Riku confronts Ansem again in the B8F Exit Hall, and he gives him another set of cards.
When he reaches the third basement, Riku encounters Zexion, who gives him a card for Destiny Islands. After clearing the floor, he meets up with Mickey Mouse in the B2F Entrance Hall. Mickey gives him a card for Twilight Town, saying that it led him to Riku.
After speaking with Naminé, Riku decides to confront Ansem once and for all. DiZ gives him the Castle Oblivion card, which Riku uses to draw out Ansem.
 Card Types
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.
 See also