Run straight forward. Go around the hall until you are faced with two sets of bars. Press the skull switch to the west, and walk along the platform to the southern switch. Push it, and jump onto the lowering platform to grab the red key. Run to the satyr switch in the northwest, and press it to regain access to the walkway. Go to the red bars in the west, open them, and run across the bridge to the western switch. Flip it, and enter the teleporter to the south. Jump into the slime pit to the north, and press the satyr switch to the northwest. Get onto the lift, and then open the red door in the northeast. Descend to the floor below, and enter either one of the teleporters. Hit the switch in front of you, and jump down once more. Go through the northern opening, and step into the hole.
Other points of interest
In the starting area, drop into the toxic pit to the north and press the switch on the western side of the green pillar. The stone wall to the west will lower to reveal a compartment containing a blur artifact and an invulnerability (sector 202).
In the area where you get the red keycard, enter the toxic pit and run all the way to the east. Take the elevator up to find a teleporter leading to a room with monsters (two imps on easy, a hell knight on medium or two hell knights on hard skill levels), ten health bonuses and a blue keycard (sector 132). This allows you to open the blue door later in the level, leading into another room (not marked as secret) containing a stockpile of 30 health bonuses, a medikit, and a chaingun, all surrounded by a cage containing three, five or six imps (skill dependent).
In the room beyond the red key bars, drop into the toxic pit and lower the bridge to reveal a switch. Flip it to reveal a compartment in the south with a berserk pack. After that, run into the compartment and open the western wall with a lightning bolt on it. This will reveal a teleporter leading to a platform with a computer map (sector 28).
The eastern end's northern wall of the hallway where the red door is accessed from has wider holes in it than the others. Opening this wall leads into the eastern imp cage in the final area; the door sector is marked as a secret (sector 228).
Secret #3 can be used to circumvent the need to lower the bars into the last part of the level. Furthermore, you do not need to lower the bridge to trigger the switch, as you can flip it while standing at the bridge's western end, no matter how high or low you are. Similarly, the switch that lowers the bars blocking the exit can be triggered before it is raised, by facing north, and pressing the "use" key while standing at the center of the platform.