(1) A very "cavelike" level. Open the entrance doorway and a warrior immediately closes in for the kill. Your best bet is to nail him with a grenade, or else you will almost surely suffer some damage. You may have to restart the level a few times before the warrior does something other than come straight at you (precluding a safe kill due to the very close range involved). (NOTE: use grenades with abandon on the first [upper story] half of this level, since there is a full complement of 4 grenade powerups [20 grenades total] to be found here! You can save some seismic charges this way, too.) Move out of the entranceway. A warrior will come scurrying at you hanging from the ceiling. These guys can be hard to take out; they won't hurt you, but they like to stay so close that you cannot aim high enough to hit them. The best tactic is to run away with the pulse rifle selected, then turn suddenly and aim up to fire a burst or 2 at the approaching enemy before he gets too close. A narrow hallway works best for this, but these enemies will usually follow in a straight line anywhere they are found. It might take a few tries, but you can do it. Now stay right. Kill the infected human in the nearby corner (he'll be the first of many, but you are still allowed to feel a little remorse here), and grab the flame thrower canister near him. Continue right (don't miss the shoulder lamp in the corner just opposite the human) and deal with another ceiling warrior. [FLOATING ALIEN TRICK: Return to this area while playing on HARD and the 2 ceiling-hangers will revive, but they will be green blotches on the floor instead. Aim down to kill them again and you will generate 2 frozen warriors suspended in midair! They will slowly ghost across the room until they hit a wall and stop!]
(2) Grab the seismic charge and battery on your way to a secret door you will dead-end at and blow it. You now need to open the newly-revealed door. Immediately 3 warriors (of the highly dangerous ground-dwelling variety!) emerge. Grenade them and move into the room. Throw the 2 switches (one has an ID tag hidden behind it) and gather up the seismic charge, pulse rifle magazine, flame thrower canister, and auto-map.
(3) Move back out of the room and stay right until you come back to the corner the infected human was at. Facing the cadaver, a secret wall is to your right. Blow it and open the revealed door. Inside are a grenade, seismic charge, acid vest, pulse rifle magazine, and derm patch. There is also another infected human. "Help" him.
(4) Move out and continue right. Blow (to reveal, then open) a second hidden door at the place where your path dead-ends. If you enter this area cautiously, you can grenade a frozen warrior in complete safety.
(5) The wall directly in front of you as you enter is a secret area; blow it and open the door to unleash 2 facehuggers and expose the pulse rifle, 2 grenades (yay!), shotgun shell, 2 seismic charges, and flame thrower within. (You'll be coming back this way, so you may want to wait until then to avoid having to kill the facehuggers twice).
(6) As you go out, the wall caddy-corner from you (ahead and to your right) also hides a secret room. Exploding it reveals another brace of 'huggers, 5 derm patches, a med kit, an ID card, and an acid vest (you can save this secret room for later, as well, if you aren't in immediate need of any of the powerups).
(7) Now continue forward and deal accordingly with 2 warriors, 1 each of the ceiling-hanger and ground-loving variety. [The FLOATING ALIEN TRICK will work on this ceiling-hanger, too.] The left wall just past the first bend in this passage is hiding a secret hallway, but wait until you return to blow it open. Keep going (your path will trace a triangle) until you reach a dead end. On the way you will kill a warrior and "help" 3 more infected humans. You will also pass an area where you can look down into an egg-laden area below. There is even a gap in the railing here where you can drop down into this part of the level, but that gap is for folks who don't find the secret areas. You can do better. Don't dawdle or get too close to the railing along this area, or you will activate some of the eggs below and maybe even release a facehugger or 2. At the dead end, blow the wall. (If you left any but the last secret for later, later has come!)
(8) Stay right (you are retracing your steps now) to the left wall just past the first bend of this triangular passage and blow it. You are now in a long hallway loaded with 2 shotshells, an ID tag, clip, derm patch, flamer canister, pulse rifle magazine, med kit, grenade, and--at the very end--a facehugger!
(9) Now drop down into the lower part of the level that you saw earlier. It will still cost you 5 health points (no help from armor here!), but you will land next to 1 egg instead of the 2 you would have found jumping through that gap in the railing you came across earlier. Work your way cautiously around this large, rectangular chamber with the pulse rifle up and ready. You will kill 3 humans of the infected variety and 16 eggs (of which 1 is well hidden!). You will also find 2 flame thrower canisters, a med kit, and a pulse rifle magazine. If you save the latter for last, you will exit this area with more ammo than you entered with.
(10) The door out is near the far left corner (from where you dropped in). Open it and back up a ways to kill the warrior that emerges (you want to kill him far enough away from the door that you don't inadvertently revive him later).
(11) Enter, move straight down the hallway to the end, and go left. Keep going to discover 1 or 2 warriors and a med kit. Turn left again (at the corner with the switch in it) and walk forward until you have activated and killed 2-3 more warriors. You will have encountered a med kit and 2 more hallways on the left in the process.
(12) Take the first one, but beware the warrior waiting at the med kit just around the corner!
(13) As soon as you kill him and move forward, the door ahead opens. If you were moving carefully at the time, only 1 of the 2 warriors inside will be activated and come running out. Kill him and then activate and eliminate his partner. You can now explore the room, which holds a shotgun, clip, flamer canister, pulse rifle magazine, ID tag, grenade, acid vest, and battery.
(14) Finish exploring to the end of that hallway, stay to the right into the next hallway, and go forward until it opens into a wider corridor (there is a med kit along the way).
(15) You are now ready to leave. Use the battery at the switch you found (it is down the corridor to your right) and turn around to walk to the end of that hall.
(16) Within the exit doorway will be 1 more warrior to vanquish before ultimate victory over this level is yours.