Thanks for the great explination! I know that a lot of things weren't there when KH was made, I mean it looks until the ending like a standalone game. Never thought of kairis heart, the castle confused me to much.
Pretty good to be here.
No problem.
You might laugh, but the first KH game in 2002 was originally
planned as a standalone game because both Disney and Square were unsure of its potential success.
Every open-ended "hint" in the first game was just placed in case the game might be successful, otherwise, any of these would have been left like it was and any concepts Nomura had thought of for continuation would have remained in his drawer.
It wasn't until the resounding success of the game that they decided to definitely expand it into a full series that, unlike Final Fantasy, has an ongoing overaching story.
Even then though, Nomura isn't planning the entire story ahead, only some vague framework concepts exist for the "Xehanort Saga" as a whole.
Actual detailed story-planning is done on a game-by-game basis, meaning that while one entry of the KH series is in development, Nomura starts to plan for the next title.
I.e. when Chain of Memories was in development, Nomura planned for KH 2, when that was in development, he planned for BBS etc.
If you're looking for wellfounded information on the series in- as well as out-of universe you're certainly in the right place. We have quite some knowledgeable individuals around here.