Original Kingdom Hearts design documents reveal many differences between early game concepts to those that appear in the final game. These include the appearance of a strange icon depicting a drastically different Destiny Islands, unused Heartless designs, and Sora having clothes similar to Tarzan in Deep Jungle.
Kathryn Beaumont, the original voice of Alice in Alice in Wonderland and Wendy in Peter Pan, reprised her role as Alice and Wendy in Kingdom Hearts at the age of 64.
A red Trinity can be found in Halloween Town that must be collected before Spoiler:Oogie Boogie is defeated, otherwise it will become impossible to collect and the player will be unable to get a 100% completion rating in Jiminy's Journal. The red trinity is located Spoiler:inside Oogie's Manor, which disappears when Oogie is defeated. This was later fixed on the Final Mix release of the game by relocating the Trinity to the ground after the battle.
After the release of the first Kingdom Hearts game, Disney was planning on making an animated series based off of Kingdom Hearts. Despite the first draft of the show turning out well, the show was never made. This was likely because they planned to continue the story with more games rather than an animation.
In Kingdom Hearts Final Mix's secrect ending, Another Side, Another Story [deep dive], Mickey can be seen wielding Sora's keyblade, the Kingdom Key, and Roxas holding Mickey's keyblade, the Kingdom Key D. These concepts were, along others (such as Roxas having yellow eyes), not used in any of the later games and they only served as a preview of what Tetsuya Numora had planned for the future of the series.
The small figurine inside Ariel's Grotto will play a melody ("Who Am I?") from the Final Fantasy VII soundtrack.
The Pride Lands were originally going to be to be a playable level, with Sora turning into a lion to fight. This idea was scrapped as fighting and walking in a four-legged body required additional programming that the team did not have time to create. The concept was later used in the sequel.
In an early trailer for the game, there is a scene showing Riku on top of Big Ben in Neverland. This scene does not appear in the final game, nor does Riku ever go to Big Ben in any way. Despite this, it is considered canon as a cutscene in Kingdom Hearts Re:coded shows that this is where Riku and Kairi went after leaving Captain Hook's ship.
The idea for the game came from an elevator ride with Shinji Hashimoto (Final Fantasy and Square producer) and a Disney producer, who shared the same building in Tokyo. They came up with a crossover as a vague production idea, and the head of Square-Enix, Hironobu Sakaguchi, later agreed to the idea.
The original data and assets for Kingdom Hearts have been lost forever. This is the reason Kingdom Hearts HD 1.5 Remix is being made from the ground up.
"Kingdom Hearts was created a long, long time ago... ...The original data was missing already. It was lost so we had to research, to dig out the original game and recreate everything for HD. It was not that easy." - Tetsuya Nomura
The ending to the beta trailer for the game showed an alternate logo. The word "Kingdom" is shown in normal golden text while the word "Hearts" is displayed in a modified version of the final logo's spiky text. The trademark heart is also missing from this version of the logo.
The enemy "Assault Rider" was meant to appear in the game but its style didn't fit any of the worlds so it was removed and used in Kingdom Hearts 2 instead.
Completing the world "Deep Jungle" before Wonderland will cause Snow White to be kidnapped instead of Alice in the cut scene which is shown after completing Deep Jungle.
In Deep Jungle, outside of the tents on a table is a pot and cup which bare a resemblance to Ms. Potts and Chip from Beauty and the Beast. These characters also appeared in the Tarzan film.
There was a planned boss battle with Riku who would be possessed by Ansem in an area titled Crumbling Island at the End of The World which was based off of Destiny Island, but devoured by the darkness.
If you enter the Hercules Cup time trial, get to Hercules, pause the game and choose to restart the time trial, it will take you back to the start of the Hercules fight and the timer will be frozen at 00'06"23.
In the Japanese version of Kingdom Hearts, the Chernabog fight does not have the "Night on Bald Mountain" music. Instead the regular boss music will play. The "Night on Bald Mountain" music was added to the international release to make the scene more true to the scene from Fantasia. The music was added into the Final Mix version of the game.
Rikku was going to have a major role in the first game. She was going to be a part of Leon's group in Traverse Town and help Sora throughout the game. She was removed because of the similarly-named character Riku, and was replaced with Yuffie. She later appeared with the rest of the Gullwings in Kingdom Hearts II.
Disney Castle was supposed to be playable. It was shown in a beta trailer but the level was cut from the final version. However, through cheat codes, it can be accessed.
The secret ending, "Another side, Another story," was not meant to be a real game. It was only there to show off ideas that Tetsuya Nomura had for the sequels. It was not until Kingdom Hearts 358/2 Days that the fight shown was playable.
Bahamut, a recurring summoned creature from the Final Fantasy series, was planned to be a summon for Sora. But it was removed from the final version. Searching through the game data, a Bahamut summon option can be chosen through the use of cheat codes. However, since there is nothing to execute when it is chosen, the game will crash.
According to some concept art released by Square Enix, Sora was originally not going to wield a Keyblade at all, but a chainsaw sword instead. He was also going to have a design with lions fur, claws, a tail and ears.
Sora was not originally the main protagonist of the series. In the beginning, Disney had desired Donald Duck to be the protagonist, while Squaresoft wanted Mickey Mouse. Series creator Tetsuya Nomura wanted neither and developed an original character based off of Disney's style. In the end, Sora was created. His yellow shoes, red shorts and white gloves are based on Mickey Mouse.
In the Secret Place, on Destiny Islands, the walls are covered in paintings which seem to foreshadow some of the earlier events in the game. For instance, there is a painting of Sora winning at the coliseum; the tree house from Deep Jungle; and Pluto waking Sora up in Traverse Town.