P.S: I think I missed two secret levels though
yep, i finally had to consult a walkthrough 15 years later to find them...! the entrance for one of them is kinda funny - it's actually a door to the left just before you hit the exit for a level, but you're usually in a rush and don't think to look to the sides just* before stepping through the exit...
it's worth playing through again later (at a tougher difficulty) just to get all the secrets. there are some funny ones.