Activate the switch in front of you at the start and you will be lowered into the northern end of a large room. Walk on top of the set of pillars and enter the teleport at the top. Travel along the walkway and make a jump towards a large circular platform. Activate both switches and wait for a long walkway to rise to the southwest. Be careful for the Pain Elementals, Cacodemons and Arch-Vile. When the walkway has risen, walk towards a small room and you will be in the northern end of another very large room. Activate a switch that is located at the entrance and collect some rocket boxes which will be useful in fighting the Cyberdemon located far away on a platform on the western end as well as other enemies. After run along the way until you are arriving at a lift on the southern end of the room. You will be in a small room with a blue door and a switch. Activate the switch and a door will open on the northwest corner of the large room. Go inside the room, climb the steps, get the blue key and return to the blue door platform. Enter the blue door and you will be transported to the Cyberdemon platform. Re-enter the teleport and exit the level.
At the very start of the level, rather than flip the switch in front of you, turn around and walk through the false wall to get some ammo and a megasphere.
Just left of the start point is a stairway of thin grey pillars leading upwards. Drop down between the pillars and get as close to the brown wall as you can, and you should land in a gate secret area with several powerups and a teleporter out.
After you reach the second large cavern area, drop into the blood below. At the east side of the pit is a radiation suit with a sunken staircase nearby (there is a green arrow pointing toward it). Flip the gargoyle switch at the bottom of this stairwell to lower a nearby section of the ledge above. Behind it is a secret room with a soul sphere and some ammo.
Inside secret #3, open the left wall to reveal a teleporter to a secret area.
After taking the teleporter in secret #4, continue forward and pick up the various goodies to register another secret.
After picking up the blue keycard, two secrets will open. The first is in the same room as the blue door...
...and the second is located in the high fiery hallway at the southern end of the second large cavern area.