Hub 1: Guardian of Steel

First visit
You appear in a small room with two flights of stairs going to your right and left, each leading to a corner-room with a bull-head switch, whose purpose I will explain later. When you'll have cleared all this section of monsters, the center pillar of each of those rooms (this whole level is quite symmetrical so no matter which way you go, you'll wind up in the central room all the same...) is in fact a door that will let you into the other part of that room (note: you can easily kill all of the monsters there by just crushing them under this door-pillar that goes down really fast :-) ). Now you should have before you a big flight of stairs runing upwards (and on your side another flight going downwards just into a small room with some monsters and goodies.

The big flight, however, will take you to the central room that has in each corner such a flight of stairs (the remaining two will become accessible later) and also two big doors. One of them (the one on the side where the stairs can't be accessed) needs the steel key, but the other one can be opened. Go through it, wait until the door behind you closes, the lift will take you down into a small steel labyrinth. There you have basically two ways, one of which is always barred. Now we get back to the switches in the first rooms I mentioned earlier. Pulling them moves that wall that's standing in your way, so you can eventually get into both corridors. For example, pulling the bull-head switch in the north might give you access to the southern corridor, or maybe it is the other way around. The western counterpart may or may not use the opposite logic later. (The cage that's between the two corridors: it is not certain at which point exactly it opens, but it will be opened when you pick up the torch in the center of the map.) Anyway, at both ends of both of those corridors you will find a switch that you should pull. You should also find your second weapon there (if you didn't already get it in the Seven Portals as you do when playing on lower difficulties). Time to get back to the Seven Portals...

Second Visit
This one's going to be very easy, as you appeared in the western part of the Guardian of Steel, perfectly symmetrical with the eastern one. So again, go through the stairs up to the room with the door-pillar (you will notice that this time, it requires the steel key to open), then through the other long stairs back to the central room and through the steel key door you saw earlier on. There again, a lift will take you down to two corridors. Go through the one that is accessible and pull the switch at its end. Then head back to the door-pillar rooms and toy a bit with the bull-head switches there, so as to move the steel bar from the corridor it was blocking to the one you already went through. Then head back to the central room and through the steel key door down and through the other corridor to yet another switch. Having pulled this one, everything that needed to be is done and you can merrily head back to the Seven Portals for the grand finale...

Third visit
You will appear right in a room quite full of wendigos so start running around and kill them all as quickly as possible, as theis attacks can hurt a lot. Then the walls of the room will slide down, revealing an even bigger room with more wendigos. When all of them will have been killed, take a look on the map to find out where exactly you are (relatively to the parts of the level you've already been to). On two sides of the room (where it touches the door-pillar room) are lifts that will take you to the door-pillar room. Go there and through the long stairs first down and then up again to the other (northern) door-pillar room. There, symmetrically, the same lifts will have started operating, letting you into another room, to the north of the central one, exactly to the opposite of the wendigo-room you appeared just now (I hope I managed to describe it clearly enough...).

This big room will be full of crushers, so explore it carefully, killing all the monsters you find on your way. On some wall right behind a crusher (again, random each time), you will find a face-shaped switch. If you have trouble finding it, just look on the map where it should be quite visible, due to the characteristic shape of switches there. Pull it and get back to the door-pillar room and through the stairs into the central room. The central area of this room will be opened now as a platform. Jump upon it, kill all the centaurs there, and then jump on the very central platform of the level, where a torch is. As soon as you get on it, a message will tell you that "stones grind on the Seven Portals". So just go back there to finally reach the secret level!