Garden of Assemblage
The Garden of Assemblage is a secret location that is used to fight against data-based versions of the members of Organization XIII, which are much more difficult than the original battles that Sora previously fought against them. The Garden of Assemblage is exclusive to both Kingdom Hearts II Final Mix and Kingdom Hearts III Re Mind, which is found at the end of the Cavern of Remembrance in the former, or as a digital recreation as part of the Limitcut Episode in the latter.
The Garden of Assemblage has an ornate mall in the style of the Castle That Never Was surrounded by a circular wall, which is itself ringed by more of the inverted waterfalls and the mountains of Radiant Garden are seen. At the center of this mall lies a computer, which can be used to bring out the data battles of the Organization XIII members.
In the data-based version seen in Kingdom Hearts III Re Mind, the waterfalls are replaced with dark data streams with dark pillars seen between them and, as this a data-based version, there is no sky.
Kingdom Hearts II Final Mix
After Sora, Donald, and Goofy clear through Cavern of Remembrance's many difficult challenges and opponents, they can reach the Garden of Assemblage at the end of the Cavern of Remembrance and the have the option to face any of the Organization Members whenever they want.
Kingdom Hearts III Re Mind
One year following Xehanort's defeat and Sora's disappearance, Riku goes to visit Squall Leonhart, Yuffie, Aerith, and Cid to have them help in their search for Sora. Cid reveals he has put together a program that would allow a data-based version of Sora to battle against the data-based copies he created of the members of the real Organization XIII, hoping that doing so would provide them with a clue into the real Sora's whereabouts. However, the plan ultimately fails once these battles have concluded, putting the group back to square one again.
In both games, the color of the available portals will start off in a blueish color but will shift to a pink color once that particular Organization member has been defeated.
In Kingdom Hearts II Final Mix, the portals in the Garden of Assemblage activate after the episode attached to their member is cleared. For Absent Silhouettes, this only requires defeating the Absent Silhouette, while for the members that are fought in-person, this requires completing the entire world episode they appear in; thus, the five portals matching the members fought at The World That Never Was do not activate until the game is completed. The battles presented here are primarily replications of the original battles against the Organization members earlier in the game (with Sora taking Roxas's place from the latter's earlier battle with Axel and only the first and final parts of the battle against Xemnas used here).
Also as there is no Save Point location in the Garden of Assemblage, it is wise to retreat to the Restoration Site as any victories will be undone if Sora is defeated and has to use a Continue if he is defeated in a later battle, and would therefore have to redo the previous battle again. Also retreating is useful if he needs to use a Save Point as there is none located nearby. Completing all the Organization XIII's Replica Data battles will earn Sora a Proof through a Treasure Chest.
In Kingdom Hearts III Re Mind, once this event begins, most of the portals for the members of the Real Organization XIII for the Real Organization XIII's Recreated Data battles will begin. The only two that are initially unavailable are the ones representing Master Xehanort and Xion which are left in a black color due to not being activated, where instead the ones for the other 11 members must be completed first to turn them on. For these particular data battles, unlike the ones from Kingdom Hearts II Final Mix, the battles presented here are original and do not replicate the battles from the main game, as the arena for the ones represented here is brand new and the battle is focused as a 1-on-1 battle against each Organization member individually with just Sora. Completing all 13 battles is necessary to complete the Limitcut Episode.
As the Limitcut Episode is a separate gameplay feature from the main game, it uses the completed save data file for the player to access which affects both Sora's level and available items. As such, unlike the Garden of Assemblage version from Kingdom Hearts II Final Mix it does have a Save Point available and a Moogle Shop to acquire more items if necessary.