Sorry for the late reply. If you found the cave secret level, then the one that you're missing is Duskwood. That one can be accessed from the Funereal Garth. You need to have completed the Ice Fissure in order to get access to it. There should be a pair of silver doors in Funereal Garth in the sewer area in the southeast section of the map. If you passed through the Ice Fissure beforehand, the doors should be open. Head up the steps from there, and you will find the portal to Duskwood hidden behind a stained glass window.
Seidolon
Peter joined
I am Seidolon, the bringer of death, destruction, and kickass WADs.