This level is a set of of interconnected square-shaped rooms in a grid pattern. The main objective is to find and destroy a set of power conduits important to the Citadel forces. But getting to them will involve solving a number of puzzles. (You may be low on #2 and #4 ammo when you first get here. But you'll be able to restock your #2 ammo in this level, no problem, and #4 in the next level.)
Step on the floor plate ahead to open the door to the first room. Note what this floor plate looks like, because you'll need to find another one like it later. At the far end of the first room is an unpowered door that's stuck half open. Also in this room is a device on the wall with a switch next to it. Hit the switch and you'll see that you've made a very welcome discovery: an ammo dispenser! This dispenses those remotely-operated land vehicles (#9 ammo). Before continuing, follow the wall toward the door you came in from and look on the wall for an irregular bright patch. There's a secret area inside with a Secret Level Access Keycard.
Remote vehicles[edit]
Go to the half-open door and launch a remote-operated vehicle into the next room. Somewhere around here is another floor plate like the one at the level entrance, that powers the doors in this room. Also, since you have an unlimited supply of land vehicles in this level, consider using them to explore farther and get rid of enemies before they even know you're here--sneaky, sneaky!
Either way, go to the second room and then to the third room, where there's a big construct. The door to the fourth room is blocked by a forcefield, and if you hit the switch "nothing happens." Jump to and activate the switches up on the four sides of the construct. Then hit the switch by the door, and the whole construct opens up. Hit the switch inside to drop the forcefield.
Back half and flooded areas[edit]
There are a few ways to go from the fourth room. The door to the right goes to the fifth room. Also in here is an overlooking booth connected to an overhead catwalk but out of reach, and a small sunken side room. To the left is a doorway leading to a big, dark, dingy area partially filled with black sludge, with additional flooded areas beyond. As with before, try using your land vehicles to explore this area. And of course, waste the bad guys along the way--and enjoy fragging those pesky Droppers best of all! You'll find that there's a long sludge-filled channel paralleling the fourth through sixth rooms and leading back to near where you started the level. Go to the sunken side room: the far wall on the left side is a secret door leading to a ledge out in the sludge-filled channel. You can launch land vehicles from here and reach the far end of the channel before they expire and self-detonate.
As an aside, go to the big flooded room; there are quite a few items in here. Off to the left is a row of tall posts in front of a grated-off channel, with items on top. Use the Mazrium platform that leads to the far end of the row to jump up to them (the nearer platform doesn't go high enough). In the corner of the room closest to the doorway you'll see a small alcove down at the bottom. This is a secret area with a Soul Capture. To get out of the sludge, go back to the doorway and you'll see that the platform is a big lift. Activate the lift to lower it.
Power conduits[edit]
When you're done here, go to the fifth room. This room connects to the long channel and is also partly filled with black sludge. There are columns, some with power conduits, irregularly spaced throughout the room. To the right is an out-of-reach doorway to the catwalk in the fourth room, and a walkway down at the sludge level leading across the room, but the other side is also out of reach. Jump across the columns to the far side of the room. (Destroy the conduits as necessary, although they don't go to anything. If you fall there's a lift in the corner by the lower walkway.)
Go through the door to the sixth room. You've now reached your objective: in this room is a big power core. This is essentially a giant jumping puzzle and is a bit of a pain--if you fall, the platform in front of the door is a lift that you manually activate. Work your way through, around and up the core. It's a bit easier to run onto those vertically-cycling Mazrium platforms than to jump to them. Destroy all the power conduits you come across. When you've destroyed the first three, your objective is complete. (The fourth, highest one doesn't seem to go to anything.) The lower walkways in this and the fifth rooms have risen into place, making it easier to get back.
Extras and exit[edit]
Go to the second doorway in the fifth room that leads to the catwalk. Watch out: the catwalk is booby-trapped! When you first step onto it, a bunch of mines will be dropped onto the catwalk and down on the floor alongside it. Fire weapons at them to set them off, and then take the catwalk to the booth in the fourth room. Hit the switch inside, and that big metal door by the level entrance will be opened. (Sometimes the trap gets tripped earlier. It might be enemies going through the doorway that does it.)
Go back around now to the level entrance, and through the big door you just opened. This leads to the far end of the big channel, where the level exit is. (You may have piloted land vehicles here or even run here earlier.) Remember to go back to the ammo dispenser and fill up on land vehicles. When you're finished, take the exit elevator to the next level.