where do i find that secret door that leads to the oozes, "feed me" head, etc.?

SPOILER

You need to go as far North as you can, into a 2x2 room (there are two 2x2s, you want the Western one) - then look for a loose rock, I think it was on the North wall.

Enjoy fighting the slimes :)

got it, thanks for the help that was driving me crazy!