I cleared all of level 3, but find no way to get to that lever that is probably opening the Iron Door, directly or indirectly.. Is it a special combination of other levers? Do I need to drop from level 2 somewhere, or climb up from level 4? I can't think of anything I haven't tried yet.
Can anybody give me a hint?

I tried that room already, placed three torches on the pillar (it doesn't have a fourth holder), and then tried to light the others in the room as a second attempt, then tried some patterns, before I gave up on that room and started to look elsewhere.

But maybe, this is supposed to be the opposite... thanks for the hint!

I am stuck here as well, cannot seem to get door open, both pillar of light looking rooms are lit up...

SPOILER

The Pillar room to the right: You must light the torches on the Walls opposite to the pillar (not the ones on the pillar itself). It will open a door to the room opposite to the Iron door where you will find a lever.