g1real: Then hope that someone else answers who knows a certain answer

Vulturas: Of course you can ask for them... The answer is always the same:
Cadomax^^
The most (only) useful "secret levels" (parallel dimensions) in the earlier games are the door at level 3 and the vortex at level 46: Level 3 is not too difficult and gives you a few more points; level 46 is harder, but the key at its end is extremely useful because it unlocks many other item families. (Okay, you haven't reached level 46 yet; but when you constantly get there, you could give it a try... But it is not very easy!)
The other ones don't bring you anything helpful at the beginning - it is better to concentrate on reaching and beating Tuber first or at least getting to leveld ~70-80 before really going into parallel dimensions...