I've got to the same part now and yeah, I'm clueless. Can't really identify any areas I haven't been to already, except that locked door with red glow in the lower levels, *just before the room where you have to move the wood planks out of the way to get down the stairs. I thought it might be something to do with the red gem, but no.
Question, have any of you beaten the red/pink orc dude? I can't remember exactly where he is but the first time I fought him I was doing piss all for damage, so I moved on. Maybe he has the key?