Cavern of Remembrance

The Cavern of Remembrance is an area of the world Radiant Garden that was added-on in Kingdom Hearts II Final Mix. It is suggested that the player be at a high level, as the enemies within are extremely powerful (rivaled only by the enemies in the Hades Paradox Cup). The cave leads to a courtyard known as the Garden of Assemblage, which has thirteen portals that contain data replicas of the members of Organization XIII.

Through the back of the Garden of Assemblage, Sora can get to the Restoration Site, in Radiant Garden, creating a link to that area. That way, Sora doesn't have to go through all of Cavern of Remembrance to get to the Garden of Assemblage.

Areas
The entrance, Cavern of Remembrance: Depths, holds a balloon in the left section, which drops Drive Orbs and also allows you to obtain items. By continuing to go deeper down inside you will eventually lead to a hall with a lot of powerful Heartless. However this can be avoided by several pathways in the room, which can be accessed using the Growth abilities Sora learns from his Drive Forms. Clearing this area requires High Jump LV2.

The rooms past the Depths include the Mining Area, in which Sora must ride whirlwinds and jump and glide to get past the machinery, and the Engine Chamber, in which Sora can get damaged by steam and Heartless, such as the Perplex, the Air Champion, the Reckless, the Air Viking, the Runemaster, flying around while being impeded by conveyor belts. Each room is separated by a section of a large area called the Mineshaft, in which Sora must use his abilities learned from his forms (Quick Run LV3 for the first tier, Air Dodge LV2 for the second and Glide LV2 for the third) to get past the obstacles. Following each obstacle is a scripted Heartless battle. The second to last area, Transport to Remembrance, is a white hall with three Nobody waves. The Nobodies are much tougher than the ordinary ones and can dish out tremendous amounts of damage, so come prepared. After each wave, your HP will be fully restored.

After that is the Garden of Assemblage, in which the data replicas are stored. Sora must activate a computer to access the data battles. The computer can be accessed any time, it's just that if there are some battles that Sora hasn't completed yet (such as Xigbar or Vexen), the data battles cannot be accessed. Note that the data battles of Organization members who are fought in the storyline of Kingdom Hearts II can only be accessed after saving clear data, i.e. after defeating Xemnas' final form once. Defeating all the Data Battles causes a treasure chest containing the Non-Existent Proof to appear.

Trivia

 * Transport to Remembrance bears an uncanny resemblance to the hallways of Castle Oblivion.
 * After defeating the three Nobody waves, if you go back to Transport to Remembrance, Gamblers do not appear at all.
 * The Garden of Assemblage resembles the Rising Falls of Hollow Bastion in Kingdom Hearts.
 * The balloons filled with Drive Orbs at certain points of the Cavern of Remembrance indicate that Sora must use the Drive Form abilities to get past a certain area. Sora needs to use High Jump (acquired from Valor Form) to get over the wall at the entrance (hence the red coloration). At the entrance of the Mineshaft, Sora needs to use Quick Run (acquired from Wisdom Form), hence the blue coloration of the balloons, and so forth. Whenever Sora enters into the corresponding Drive Form, the balloons light up.
 * Of all the members' weapon symbols, Xemnas's is the only one that doesn't depict his actual weapon. It instead depicts the shield used by his Dragon form.