(1) Figuring out the switches on this level makes it one of the hardest in the game. Other than that, there are really no secrets. Open the door to each room and explore it--that's all there is to it. One very important note: for some reason, the adult dog aliens encountered on this level are several times harder to kill than those anywhere else in the game. It will require 26 direct shotgun blasts or 22 pulse rifle bursts. Use a grenade or seismic charge on each adult dog to kill it quickly and to prevent its return when you retrace your route.
(2) Now to the level itself: the room you start in holds 2 derm patches, a shotshell, and night vision goggles.
(3) Move out the door and you are looking down the large, primary corridor of the level. You will reach 3 rooms to your left with a very short corresponding corridor across the hall from each. At the first of these corridors (to your right) an adult dog alien may charge you. Grenade it and continue. Don't miss the derm patch in this corridor as well as the next one. Of the 3 rooms, the second is the only one open right now. It holds 4 patches.
(4) Near the end of the main hall, grab the derm patch and take the third corridor. Turn right (there is another derm patch here) and go all the way down this long secondary corridor (parallel to and beside the main hallway) until it turns left. There may be an adult dog along the way. There are 2 derm patches just short of the T junction you will reach shortly and 1 in the junction itself.
(5) Explore left and you will find another patch at a closed door.
(6) Explore right and you will end at a pair of closed doors; on the way you will find a pulse rifle magazine (activate your night vision goggles here), an adult dog, and 3 derm patches. In the only open room you will find here (on your left) are 2 flame thrower canisters and a shotgun shell.
(7) Now reverse your route to the point where you first entered the secondary corridor. Work your way forward by following the "bread crumb" trail of 3 derm patches (ignore the part of the corridor that goes around a room to your left for now). You end up at a T junction.
(8) Turn to your right and move ahead, picking up 2 more patches. Ignore the left corridor you reach in a moment and continue to the end of this one. Waiting for you there are an adult dog, battery, med kit, and pulse rifle magazine.
(9) Now turn around and stay right, entering the corridor you ignored a moment ago and grabbing the 4 derm patches and flamer canister on the way. You may have to kill an adult dog first, though.
(10) Exit VERY carefully. There are an egg and infected human just outside the end of the corridor and if you are not cautious you will release a facehugger or chestburster. As soon as you leave this corridor, a door closes behind you at the nearest junction and the words "Door activated" appear. Now enter the large switch room, pulse rifling all the eggs and infected humans as you go. Also to be found in this cave-like room are 3 derm patches, a flame thrower and extra canister, and shotgun shell. At the far end is a short corridor with a battery at the entrance and, if you didn't kill one just prior to entering this room, an adult dog at the end (by a closed door and a med kit). Kill the dog now or he may escape when you open the doors, leaving you with a deficient score for ALIENS at the MISSION ASSESSMENT screen. Now for the switches: Starting from where you FIRST enter this room, there are 3 switches to your right and 2 to your left. The 3 on the right we'll call switches 1, 2, and 3 (with 1 being the closest to you and 3 the farthest); the 2 on the left we'll call switches 4 (next to the flame thrower) and 5 (by the shotshell). NOW ACTIVATE SWITCHES 1, 3, 4, 2, AND 5 IN THAT ORDER. You have just opened ALL the remaining rooms on this level.
(11) Now to explore them. Go down the far corridor (the one past switch 3, that had the battery at its mouth); there will be a new adult dog at the end requiring a grenade. Move forward, passing the single open room you explored earlier. As you near the end of the very dark corridor just beyond, you'll see another adult dog appear on your motion tracker. Grenade him and stay right into the room he just vacated. Within are a derm patch, shotgun and shell, and acid vest.
(12) Exit and stay right; just past a corridor on your left is another opened room on the left side holding flame thrower fuel and a smartgun.
(13) Exit this room and stay left (continuing in the direction you were headed when you reached this room). You will end up at yet another T junction, where another dog will attempt to ambush you. Turn right to grab the patch and then continue left. Another adult dog is ahead; after eliminating him take your first right. An adult dog will attack. After killing him, you will find a pulse rifle magazine on your immediate right in a very small recess and a shotshell and opened room (the source of that last dog) to your left. The room contains another shotshell, night vision goggles, and the auto-map (better late then never!).
(14) Come out of this very small room and move straight forward to the end of this hall for a derm patch and shotgun shell. You are now only a few feet from the next room you want to explore, but since the door right in front of you is closed you will have to backtrack a ways to get to it. Go back down the hall you are in and turn left just after you pass the very small room. Now stay right from here for quite a ways (skip past the 1 open room you encounter). You will pass the 3 short corridors (connecting the primary and secondary hallways) to your left and eventually arrive at a T junction. Turn left. There is an open room on your immediate left, and out will charge another adult dog. Just past the room is the continuation of the corridor and another dog. Kill them both and examine the contents of the room: derm patch, 2 flamer canisters, shotshell, and pulse rifle mag.
(15) Now backtrack to the 3 short connecting corridors. Only the third one (in this case, the first you will reach) is open. Go through it into the main hall and carefully approach the room it faces (room number 3). Inside is another adult dog; if you approach cautiously, you can lob a grenade at it for an easy kill before it is activated. Grab the 2 grenades (running low, were you?) and body armor and proceed to room number 1.
(16) Within are another dog, a seismic charge, and an adrenaline burst.
(17) You have now fully explored all the rooms in the level. However, you did not open all the hallway doors as your mission requires. A quick run back to the switch room, activate a few panels, and you can exit. To wend your way back to the switch room from here, simply go to the other end of the primary corridor and go through the short access corridor you came in through. Now all you do is stay RIGHT all the way back. ACTIVATE SWITCHES 1, 3, AND 5 IN THAT ORDER.
(18) Go back out the way you came in and just stay left all the way to the entrance/exit room. Activate the switch to open the exit and you are done. [BUG! There appears to be a sequence-specific bug on this level. If you have trouble achieving 100% for ALIENS at the end-of-level ASSESSMENT screen--even though you darn well DID kill them all--try reviving and re-killing an extra facehugger or two in the switch room (you may want to deliberately cause 1 or 2 eggs to release their contents in order to accomplish this). If you are playing at the EASY or MEDIUM difficulty settings and the problem persists, follow alternate routes to determine the "proper" order for dispatching the aliens.]