Back to Basics - Secret Level

How do you get there? I have 5/8 secrets and I've only seen the IV switch. Where are the other switches?

Look them up in either very-secret areas or just regular places.
I remember the "I" one being on the edge of the central building. I straferan alongst the RL pillar and got there, then headed right and in the very corner, I pressed space against the wall. A satyr face appeared. Pressed space again.
The "II" one is on the stairway of the Soulsphere secret (?), but I'm not sure whether it was a secret or even featured a Soulsphere, the only thing I remember about it that originally it was blocked by metal poles.
The "III"... maybe in a hi-tech themed secret with grates opening to a compulsory place.