Castle of Illusion's charm saves it from its poor controls.
Playing Castle of Illusion feels a little odd, what with Sonic the Hedgehog, and so Castle of Illusion is a much slower, and unfortunately, simply not as fun a game as Sonic, but not entirely due to the lack of speed. The controls are what really stop Castle of Illusion from reaching its full of potential. For starters, they are horribly slippery, leading to many frustrating collisions and deaths. Further control issues arise when trying to jump on an enemy. Instead of going for the simple jump on them mechanic a la Super Mario, the developers make you press down on the D-Pad as well. Now, while you do get used to this in due time, it will lead to much irritation early on thanks to many deaths which could've easily been averted.
The aforementioned lack of speed does bring some problems with it as well, such as the fact that dying means lots and lots of slow, arduous backtracking. Some of the levels offer unique challenges, such as platforming on the ceiling, jumping on enemies to get to higher ledges, and moving platforms in the shapes of leaves, but these don't excite for long.
Castle of Illusion nicely carries its Disney roots through with imaginative levels. Forests, a Toy Land and various other realms give the game that special Disney touch. The animation is superb for such an early title, and is almost on par with Mickey Mania, a game released 4 years later. The only downside to the lovely graphics is the frame rate, which can take quite a beating whenever Mickey is swinging on a vine, but that's once, on the first level.
Castle of Illusion's sounds are happy and perky, and are similar to those seen in a Disney movie, with some majestic scores, and booming effects which help to add to the atmosphere of the levels.
Castle of Illusion is a very easy and simple game, and so you'll rip through it in a couple of hours, and due to the slow nature of the gameplay, replay value is low, as once you're through, there's very little reason to plod through the game again.
Closing comments: Castle of Illusion definitely has the charm which comes along with a Disney production, but the off controls and lack of replay value hurt the game, and if you're looking for a "simple, retro" platformer, then you can do a lot better.