Ariel was originally meant to be a Princess of Heart, but in order for her to be captured by Maleficent, the story portion of Ursula giving her legs would have had to take place first. It was already too late in production to add this, so the producers chose Alice as a Princess of Heart, which garnered fan outrage, due to not staying true to the Disney Princesses. However, after finding out the troublesome production of the game, the fans forgave the game developers.
The idea for this game came about when Producer Shinji Hashimoto met with a Disney executive in an elevator; Square and Disney had worked in the same building in Japan.
It is possible to complete the game without visiting the worlds Halloween Town and Atlantica. It's also possible to leave Monstro without having to complete that level too, as Monstro does not have a keyhole.
If the player completes Deep Jungle before Wonderland, the scene after sealing the keyhole is different. When Maleficent mentions the Princesses of Heart, Alice is normally shown. However if the player hasn't completed Wonderland yet, Snow White will be shown instead.
Sora's role as the hero was originally meant for Mickey Mouse. You will notice that Sora wears large yellow shoes, red shorts and white gloves. Disney wanted Donald Duck to be the main character, but Tetsuya Nomura wanted a human protagonist.