There we go. I tried playing Immortal Ruins again yesterday, and I found what I had been referring to.
I think it was the Old Warehouse. On the good side, it really looks terrific 8-) But on the other hand, what a killer!
There are lots of hidden parts behind which Jack has to walk around. Of course, the ways being hidden, you have no idea where you should go - left, right, up, down, should you jump and grab a vine, is there a lever etc. Fair enough. The difficulty is that if you make the wrong guess you may well fall to Jack's doom, which is a bit unfair. (Also, some monsters feel a bit too difficult to avoid - I'm thinking of pairs of scorpions here - but this might just be me not being careful enough).
Hidden bits should be either fool-proof, or a bit less hidden so you can keep track of where Jack is, or just have any sort of clue which may help you around (mirror image for example

, or having passed the exact same room, but visible, just before, etc.).
This is not easy to do and the difficulty of such an area is very hard to assess for the designer.
Actually the mines inside the mountains (above the Warehouse section) were also a difficult hidden part, but for some reason I felt it a bit less hard. Maybe it was shorter, maybe it was easier to keep track of Jack (maybe there were fewer pitfalls).
[smiley=beer.gif]