(1) This is one of the more fun levels of the game, with a great deal of variety to it. However, that does not mean it is easy: there are NO grenades available here, so you will want to conserve them--though there are some grenades on the subsequent powerup level.
(2) Open the entrance door and proceed. To your right you will see a synthetic standing on a pedestal lift within a sunken courtyard. This humanoid will drop a smartgun magazine every time he dies. Wax him and turn right; now move very cautiously into the room ahead. Shoot the egg right in front. DO NOT grab the battery on the central table as the consequences would be dire. There are 2 eggs in alcoves to your left and right, and you have to walk around carefully in order to get a shot at them without causing them to release the facehuggers they contain. Destroy a fourth egg on the opposite side of the table and you are ready to get the battery. See the 4 adult dog aliens in the cages? Touch the battery and the gates drop! What you want to do is back up into the battery. As soon as the gates open, dash forward past the entrance doorway and turn around. Any dogs following you will have to move down the narrow walkway, making them fairly easy targets for grenades. Not all the adult dogs will follow at once, so you will have to creep forward until you see them show up on your motion tracker. You want to be certain that you have vaporized them since you will be returning to this area repeatedly. This done, move back to the room. Get the auto-map and med kit on the table and activate the 2 switches.
(3) Do an about-face and move down the walkway, taking the first right (in front of the entrance doorway). The synthetic ahead doesn't drop a powerup when he dies, so use a seismic charge or grenade on him. Ahead and to the right you will see another synthetic on a pedestal lift. Like his twin brother, this one also drops a smartgun magazine upon being killed, so just use the pulse rifle or smartgun on him.
(4) Now turn around and stay to your right. Proceed to a stairway with a synthetic waiting on the other side. Use the pulse rifle or smartgun on him until he gives up his smartgun. You can now run back and forth between this synthetic and the ones on the lifts, repeatedly killing them and making them drop their smartgun powerups. When you have topped off your smartgun and have dropped a couple of magazines on each of the lifts, grenade the 2 lift-dwelling synthetics so they are not a nuisance later.
(5) Now head back to the stairs with the synthetic that drops the smartgun and enter. Don't grenade or seismic this guy since you will be in need of more ammo on your way out. Go down the other side of the stairs and an adult dog comes after you. Smartgun him and proceed to the end of the corridor. Opposite the door is a secret wall; blow it and you find a med kit and seismic charge.
(6) Now open the door. Within are 5 adult dog aliens. You can kill some by blowing up the barrels; the rest will require smartgunning. Work your way to the far end of the room and flip the pair of switches.
(7) Head out, but before leaving open the secret room here. It is behind the wall to your right when you first enter. Inside are 5 eggs, a med kit, a seismic charge, and a pulse rifle magazine.
(8) You can now leave, although some of the adult dogs may have revived and the one you killed outside the room will definitely be waiting for you. Exit and use a seismic charge on the synthetic on your way out. Turn down the walkway right in front of the entrance door. You will reach a series of bends in the corridor as well as some stairs. Open the crates you encounter along the way for a pair of med kits, a battery, and 3 derm patches. At the bottom of the last stairs another adult dog is waiting for you. Use a grenade on this one.
(9) Now move forward to an open area with numerous furnaces visible at the top of some steps. Shoot the nearest eggs up here and the 2 closest adult dogs, but do not explore any more here just yet. Go back down the steps and, facing the wall to your left, launch a seismic charge at it.
(10) This is probably the largest secret area in the game. Just inside are 2 more adult dog aliens needing smartgunning. Continue forward into the pedestal area and kill the 2 remaining adult dogs.
(11) Activate the first switch (by the single pedestal) and move to the switch between the other 2 pedestals. As soon as you activate it, a section of the catwalk behind you dividing the 2 sunken courtyard areas drops. Quickly run over it into the next area.
(12) There are 2 adult dogs here to be killed. If one or both ran across the lift when you dropped it, you will have to deal with them first and then drop the lift for another try. At the end of this new courtyard is a med kit and switch. Activate the switch to drop the catwalk section back down and run back quick--this is one of the fastest lifts in the game. Upon your return, you may have some reanimated adult dogs to deal with.
(13) Now, with your back to the catwalk dividing the 2 courtyards, face the wall between the single and the double pedestals. Throw a seismic charge to open the secret. Within lie 2 facehuggers and a pair of shotshells. Another seismic at the back of this room reveals 3 huggers, 1 seismic charge, and another shotshell. But wait--there's more! One more charge to the rear wall of this room unleashes an adult dog. After dealing with him you can grab the 2 pulse rifle magazines he was guarding.
(14) There is one more secret here. Approach the secret wall you blew to access this whole area. With the opening off your right, throw a charge at the brownish (not greenish) wall in front of you. An adult dog lies within, as do a pulse rifle and seismic charge.
(15) You can now proceed to lower the pedestal lifts and move on to the furnace area. To drop the far pedestal, face the left or "West" side of it (in terms of your auto-map orientation) and simply walk up to it. You should find a stack of smartgun magazines (from repeatedly killing the synthetic perched here) and a med kit. The next lift drops in a similar manner. Stand beside the switch that activates the catwalk lift between the 2 courtyards with the wall at your back and the switch to your left, facing the pedestal. Walk up to the nearest side and down it comes. The only powerup here is a pair of night vision goggles. To access the single pedestal, simply run between the pair you just activated (you will hear the sound of a lift moving) and then dash to the lift and grab the body armor and smartgun magazines waiting for you. You can ride any of the pedestals back up for a bird's eye view of the surrounding area and drop back off without taking any damage!
(16) Now head back for the furnace area, dealing with any revived adult dogs as you go. The 2 dogs you killed here earlier will be revived, and a third lurks to your right. Work your way carefully through the area, watching for eggs and staying as far as possible from the furnaces. A close encounter with the latter drops your health 5 points! Make sure you get all the eggs as you move toward the far right corner.
(17) Blow the secret wall (again, the brownish section) to access a doorway into the final room prior to the exit area. Two adult dogs will charge you as soon as the door lifts. Kill them and move inside.
(18) As soon as you activate the switch to your right, 3 facehuggers dart out of wall recesses to your right and a door to your left opens to release 3 more. Try to avoid being facehugged as you will be assaulted from both sides. Having survived, enter the new room for 2 med kits and a seismic charge.
(19) Now throw the switch opposite the first one to open the exit door. Kill the pair of 'huggers that pop out from the wall behind you and you're on your way out.