Sega has preserved that difficulty in the remastered version of the game, as seen on the floor of E3 2013 last week. Sega Studios Australia has teamed up with the original game's creator, Emiko Yamamoto (who still works for Disney in Japan), to recreate the game in 3D and add new elements.
For the most part, Sega has tried to recreate the feel of the original Castle of Illusion, if not exactly the look. One of the most important things about the original game, Sega's rep told me, was that Mickey was very animated as a character (especially considering his Disney heritage), and so the new 3D version is also similarly active. When idle, he stops and looks around curiously. When near a ledge he pulls himself short and steps back, and even while ducking he flings his body down and then glances around to spot trouble.
The enemies have also been visually upgraded, but not as extensively as Castle of Illusion's backgrounds. Going from simple 2D to a "2.5D" side-scroller has enabled Sega to really fill out the backdrops, and make the magical castle's settings even more intricate. In addition to some really beautiful background environments, Sega's done a nice job of adding more animated touches. At one point, you pass a gothic painting of Goofy, and the eyes follow you as you go by in true Haunted Mansion fashion.
The 3D graphics also made it possible for Sega to add new segments to the game. One such segment has Mickey traveling across a series of platforms moving in and out of the walls. At first, he simply travels across a 2D hallway, but then the platforms curve around towards the screen, and Mickey has to make his way down a tower that curves forward as he descends. The new segments feel very faithful to the original, but they definitely modernize the game's pacing a bit and justify the new technology used.
The music is also worth a mention. There wasn't much to hear in the one level I played, but the new orchestral soundtrack, put together by composer Grant Kirkhope (of Banjo Kazooie and Goldeneye fame) was particularly ear-catching. The stirring tunes, mixed with the colorful graphics, add a whole lot of magic to Castle of Illusion's scenes.
There are new hidden areas and new hidden items for players to find, and a Sega representative told me those collectibles will be used to earn "unlockables," though those remain secret for now.
Castle of Illusion looks like a fitting tribute to the old game, along with plenty of revitalized areas combining newer techniques with the flavor of the original. Just don't expect a walk in the park.