Castle of Illusion was indeed great, as was the sequel World of Illusion. I have both original Genesis releases, complete with box and booklet, from my childhood. I loved them then and I love them now.
It wasn't until recently that I discovered that SEGA AM7 developed them! What a surprise!
It is cool that the 3DS version will be the fourth Illusion title (don't forget "Land of" for the GG and MS). Shame it's not deveoped by SEGA, but given how rights have changed over the years, it's cool. I'm actually really excited for the handheld version more than the HD version now!
I might do a weekly five on the "Illusion" series this week. I think they deserve it!