Run through the initial area to the northwest. Open the large door with the skull on it and continue into a room with two more skull doors. Grab the megaarmor if you want to and enter through either skull door in front of you. There is a rocket launcher in this room along with a few crates of rockets, and a skull switch. Press the skull switch and you will be lowered into a large room with many pillars. Beware, there is a huge number of imps who will swarm you - it is recommended to use a BFG9000 or a rocket launcher to kill them. There is also a cyberdemon in this large room, so use caution. Various skull switches around the room will lower sections of wall and pillars containing more imps and various powerups, and also will give you more maneuverability to dodge the cyberdemon's rockets. Near the north edge of the map is a skull switch that raises a set of stairs - go up these stairs and kill the imps at the top, then run across the pillars to the south. To the left of the last pillar is a teleporter between two candles on the corner of a wall - this will warp you back to the beginning of the map, where the previously raised teleporter in the center of the first room now has stairs leading to it, allowing you to exit the level.
Move forward to the demon face door. In this room, collect the rocket launcher and ammo.
Equip the BFG9000. If you do not have one, equip a rocket launcher. Press the button (the room will start to descend into the tomb) in the back of the room and run back to where you came in. You will be unable to leave the room, but going back here will give you some space between you and the imp horde you are about to face.
When you are nearing the bottom of the tomb, fire your weapon at the imps and try to clear them out as fast as possible. Pay close attention to the two openings as a cyberdemon will be coming toward you, usually in the western door (to your left). If there are still some leftover imps when the cyberdemon comes, abandon killing them and run into the hallway he is not in. Shoot the cyberdemon with your best available weapon. By quickly entering and exiting the room you started in. When he gets too close for comfort, retreat from the hallway you were in and go to the other hallway entering the room you started in. Repeat this process, drawing the cyberdemon from hallway to hallway (keeping him funneled in the room you started in) until he is dead.
From the map start, descend the stairs and ascend the first set of stairs to your right. Open the northeastern canvas on the southeastern wall to reveal a switch. Use it to open a passage (sector 173) in the northeastern wall. Enter it to find ten health bonuses, a plasma gun and a computer map.
On descending into the tomb, go west until you hit a wall, then follow north to the corner (sector 212) to your left. Note that it is bordered with metal girders, lower it to find three armor bonuses and a berserk pack on it.
Push the wall inside secret #2 to bring down the last secret in the game: an invulnerability sphere (sector 213).