At the start of the map, turn around and push the wall to reveal a plasma gun. (Sector 27)
After using the first elevator, you see an unreachable blue armor on the left side. Simply shoot at the bars behind it. (Sector 6)
In the room with the blue and yellow door, go through the door which doesn't require any key. Follow the path just after the lift. Shoot the eastern lantern, so the switch behind the northern lantern becomes reachable. Actiavate the switch, so the BFG-9000 and invisibility sphere in the following room become accessible. (Sector 144)
In the - so far explored - most northern part of the map is a room which contains some mancubi as well as one (on easy, two on medium and hard skill) cyberdemon. On the eastern wall is a switch, which reveals another switch, that reaveals another switch, and so on. The switch on the south western pillar also opens the northern side of the central northern pillar (Sector 173), which contains two boxes of shells.
The switch on the north western pillar also opens the southern side of the central southern pillar (Sector 175), which contains two medikits.
West of the exit, you see a blue armor. shoot at the bars behind it to open a part of the wall. Pass that, kill the mancubus and open the wall behind it to reach the blue armor. (Sector 204)
East of the exit, you see a bulk cell (there are six bulk cells at least on hard skill). shoot at the bars behind it to open a part of the wall. Pass that, kill the mancubus and open the wall behind it to reach the bulk cell. (Sector 209)