At the beginning of the level, the southwest alcove with the ammo box has a trio of metal panels on its rear wall. Push on it, and a lift will lower to reveal a berserk pack. (sector 64)
Within secret #1, look to the west to find another trio of metal panels. Press on this one as well to find five health bonuses. (sector 66)
In the first metal room accessed from the hallway south of where you start, you can look in the northeast alcove, after the four Imps are released, and find a switch on a wall in an inaccessible room. After hitting the switch in the center of the room you are in, shoot this unreachable switch, and a door will open. Quickly run into the computer room east of this area, head into the southwest passage, and look through the window to see another switch. Shoot this switch, and then backtrack to the central, metal room to the window you looked through before to see another switch. Each of these last two switches must be shot within nine seconds after the doors that block them are opened. When all three switches are successfully triggered, return to the eastern computer room, and go to the southwest passage again to see a teleporter. Enter it to be taken into the previously unreachable room, where you can get three stimpacks, combat armor, a box of shells, a bulk cell, and a chainsaw. (sector 134)
In the western metal room (west of the central one where you shoot the first and last switches in secret #3,) the switch you must trigger to lower a lift and access another switch out of the room will lower another lift opposite the other one, behind a fake wall. Walk through the wall to get four stimpacks. (sector 217)
After teleporting to the southeast side of the red skull area, enter the teleporter you landed on to taken to the southwest corner. Look to the left to find a differently-textured wall with an arrow pointing to it on the floor; press on the wall, and it will lower to unveil a rocket launcher. (sector 304)
In the green hall after the cavern following the spiral staircase, one of the demonic faces is bright and flickering. Press on it, and a closet will open in the east cavern, containing a soul sphere. (sector 448) Be careful, however; once you grab it, you will be attacked by an Arch-Vile.
The moment you enter the cage where the yellow skull is held, a switch will reveal itself west of the bloodfall. Make your way to the switch as quickly as you can, and press it. Closets will open in the southwest hall. The northwest one contains a light amplification visor... (sector 510)
After getting the yellow skull, backtrack to the northern, brown room to find combat armor sitting on a switch pillar. Press any of the switches, and although you will get the armor, you will be trapped in the center of the room for 36 seconds. Fight off the enemies that appear until you are freed. At the same time as you are freed, two alcoves will lower in the southeast corners containing a plasma rifle and a bulk cell. Both of these count as one secret. (sector 526)
In the outdoor section behind the yellow skull door, going down the south side of the northwest staircase will temporarily open a switch at the northeast side. Quickly shoot the switch through the window, and an alcove will open behind you, containing two medikits, security armor, and two boxes of rockets. (sector 631)
After you get lowered into another room at the southeast of the aforementioned outdoor section, the stairway in front of you has a trio of metal panels right near the window. Press it to lower a tunnel containing five health bonuses. (sector 641)
Get everything in secret #11, and return to the room you came from to find a huge alcove has opened in the south, containing two bulk cells. (sector 642)
After the three keys barrier in the western, green hall, there are two bodies hanging on the west of the tunnel; one is right side up, the other upside down. Press on the one that's right side up, and a door will open at the east of the tunnel very briefly; immediately dash under it, and head downstairs to find a teleporter. It will take you to the megasphere in the center of the next room. (sector 699)
At the northeast of the map, after opening the three southern alcoves in the northern room, look into the center one, which has bloody remains in front of it. Open the back wall to find three cells. (sector 753)
Enter secret #14 and a wall will lower at the east side of the room. Run through the opening before the wall raises again to enter a metal hallway. Walk through it until you come across two satyr faces on a wall; approach them, and the wall will rise to enable access to a BFG 9000. (sector 761) Beware, however, that when you obtain the weapon, enemies will be unleashed into the hallway.
This level cannot be completed in no monsters mode outside of using the noclip cheat, as the presence of a Baron of Hell (thing 455) is required for the three-key barrier in the northwest, green hallway to open.