(This level not only has a great layout and architecture, but also some of the most interesting and advanced interactivity. Out of the DLLs that provide higher-level functioning and interaction, this level's is the largest--hardly a surprise.)
You begin in a dimly-lit room with plumbing along the floor. A window provides a glimpse into an area not yet accessible. Follow the pipe down the corridor, which leads down to a tall area divided into sections, with three biogen tanks. These tanks supply biogen fluid to spawning chambers past that window in the first room.
Biogen tanks[edit]
Your first objective is to drain these tanks. Go to the far end of the tanks room, and up the sloping corridor. Go left at the junction to a series of overlooking control rooms. In the far room are three switches on the wall. Hit the three switches, which partially drains the biogen fluid from the tanks. In each tank you'll now see a switch that was submerged before. Go to the tanks and hit the switches inside, which will drain the remaining biogen fluid and complete this objective. (Also, jump out the window in the room with the switches, and go along the pipes. There's a Big Buddy on the pipe, and the overlooking room opposite the tank has items.)
Go back up the sloping corridor, but this time continue straight at the junction. Farther ahead the corridor has caved in, but you can see that it continues beyond. You will have passed a door on your right; go back to the door and into what looks like a laboratory. In here are several octagonal alcoves along the outside. Go to the one on the opposite side of the room, to the left of the other door. Activate the left-hand wall in this alcove (note the misaligned texture) for a nice secret area with good items. (It can be gotten into from the alcove on the opposite end too.) Both the other door and the grating with the full-health inside lead via corridor or air duct to what we'll call the Ramps Room.
Ramps room[edit]
After you're done exploring the laboratory, take either way you choose to the Ramps Room. Despite its simplistic appearance, this is probably the best interactive spot in the game, mainly because of its elegance. You'll be in a large, tall rectangular room that's mostly empty, with various doors and air vents up on the walls. Look around, and you'll find five switches. You'll see a ramp leading up to the door closest to where you came in, plus there are constructs on the floor by the other doors and along the walls at either end of the room. These are also ramps and ledges, in their lowered position. Play with the switches and you'll see they raise and lower various ramps. You'll probably get a "nothing happens" message at times, revealing that there's a bit of a puzzle here, but it's simple: each switch operates one ramp, and you can have up to two ramps raised at one time. You can figure out which switch goes to which ramp; don't want to spoil the fun too much. But it's consistent, not random.
(Take a look at the ramps raising and lowering, especially the ones at either end. This was especially well done, with the slope as well as the height seamlessly lining up the entire time. Also, the three diamond-shaped steps prove that it's within the game's capabilities to have movable adaptive polygons, although this is the only instance of it being utilized.)
For reference purposes in here, this will reference compass directions. The door/vent and ramp where you came in here is the northeast door and ramp; the ramp/ledge along this end of the room is the east ramp. At the other end of the room is the northwest ramp and door, and the west ramp. The door above the diamond steps is the south door.
South door[edit]
You'll see the south door is unpowered. You can take care of this now, or after you complete your second objective. To power the door, first raise the northeast and east ramps, and go to the air vent at the far end of the ledge. (There's a Deep Radar Tracking floating high up in the air you can jump to from the ledge.) Follow the air duct, which leads back to the corridor from the biogen tanks, but on the other side of the cave-in. Go from here to a room with a bunch of partitions. In here, near the far end, is a switch: hit it to power the south door in the ramps room.
Now, for that door in here that "will not open the normal way." This is probably the most non-obvious secret area in the entire game, and most difficult to initially figure out. To open the door, go back through the air duct. Just inside the duct, as you're leaving the corridor and going back to the Ramps Room, you'll run through an invisible trigger which opens the door. Listen while you walk down the air duct and you'll hear a door open in the distance. That's the door to the secret area. There are two Death Spheres inside; the switch opens the door from the inside.
Skeevin spawn vats[edit]
In the Ramps Room, raise the northwest and west ramps, and go up to the door. First, go to the air vent up and to your left, where there's an Invulnerable in the caved-in air duct. Now, go through the door and to the Skeevin spawning lab. A barrier slams down behind you--it's a trap! But don't worry, it's not difficult. You'll see four switches in the middle of the room. You can hit these now, or they'll activate on their own over the next several seconds. These will release lots of Death Dart cells for you to collect. Well, OK, it releases the Skeevin from the spawn vats, but you'll have long-since found them to be a convenient source of ammo more than a hindrance. After you're done, hit the switch by where you came in to open the barrier and turn on the lights. You'll also notice this is the room past the window you could see from where you started the level.
An important side excursion: In the spawning lab, look for an air vent behind the vats. Follow this air duct, which is beyond the caved-in section you saw in the duct outside, to a side room overlooking the Ramps Room through a grating. Look behind the console for a bright section of wall. There's a secret area inside, with a Geosurvey and a Secret Level Access Keycard. While you're here, look at the security camera on the console: this is one of the Roto-droid spawning tanks, where you're going next.
Roto-droid spawning tanks[edit]
When you're done here, go back to the Ramps Room, and if you haven't already, follow the above directions to power the south door. Raise the diamond steps, and take the south door. In here is a tall room with a raised central area, and three large doors plus four small doors, none of which will "open the normal way." Take the lift to the central area and step on the floor plate. This reveals three shootable switches on columns in front of each of the three large doors. Each of these shootable switches will briefly open one of the three doors, inside which are the Roto-droid spawning tanks. But the switches' locations don't correspond to their respective doors; experiment to see which switch opens which door. Before continuing, shoot all three switches within a few seconds, which will briefly open those four smaller doors in addition to the large ones. There are compartments inside with items; each counts as a secret area.
Now for the last objective. Inside each of the spawning-tank rooms is a switch. Hit the switches in the three rooms. In the center spawning-tank room is a lift to a small room, with a floor plate blocked by bars. After hitting the three switches, the bars are opened. (Save the center room for last, for convenience.) Go to the small room and step on the floor plate to shut down the spawning tanks and complete your objective. When you do this, all of the Roto-droids inside the glass chambers will be released, so have weapons ready. (You can ignore the freed Roto-droids in the other two rooms if you're not after every enemy.) There's a teleporter in the center spawning-tank room, which exits the level. This teleporter only works after you've completed the three objectives, although it always appears activated. After you're done enjoying the sights and all that, take the teleporter to the next level, which also has a great layout and interactivity.