1. Grab the supplies then continue through the door. Note that your bugbait will not work anymore, at least for a time.

2. Descend these stairs then enter the hole in the wall ahead. Ready your pistol first, as there is a head crab inside.

3. An automated turret can be seen firing at antlions on the left. Ascend the stairs on the right, where another turret is hard at work.

4. Enter this office to use the health and energy terminals. You can also change the view on the closed-circuit TV, spying a shot of G-Man (inset). Continue to the next door on the left and ascend the stairs.

5. Push the door aside at (1) to find an ammo crate. Continue through the door at (2), after tipping over the turrets with your gravity gun.

6. Descend some stairs and a ladder, then continue up the stairs visible here, careful to avoid the barnacles on the ceiling.

7. Ramp up these stones to the floor above. The turrets you recently toppled will be behind you.

8. Ready your Overwatch rifle before approaching this door (inset). A group of head crabs attacks from the ceiling as you approach.

9. Pass through the door then quickly drop below here, as turrets will be shooting behind you.

10. Two turrets are at the top of the stairs. Toss grenades to topple them, or quickly ascend the stairs to get in range with your gravity gun.

11. Climb the stairs, then pass through this corridor to load the next area. Ready your pistol.

12. Ouch, missed. Take out this troublesome head crab then find grenades in the crate. Use your gravity gun to clear a path to the door ahead.

13. Drop to the showers below and an antlion bull appears. I tried throwing barrels and crates at him with the gravity gun but found that emptying my Overwatch rifle (including secondary fire) then following up with my SMG worked much better.

14. The small holes in the wall don't afford any protection, as the antlion bull will just bust through the walls anyway. This does make it easy for you to keep up speed as you kite through the large circular area. The bull has distinct charge and retreat modes; be sure to run while he's charging.

15. Eventually the bull will fall, and you can once again use your bugbait to summon antlions to do your bidding.

16. Use your gravity gun to clear the debris from this door.

17. Just past the door is your first opportunity to summon your refound pets. Toss the bugbait at the troops ahead, then take cover as they do their work. They'll even topple turrets.

18. Once the area is clear, use your gravity gun to pull the plug on this little operation.

19. Whatever they were doing to this alien in the barber chair, he ain't moving now. Flip the switch at the red light through the next door. This opens the gate around the left corner.

20. The supply crate in the corner holds ammo for your pulse rifle's secondary fire. Continue up the stairs.

21. Notice the claymore mine on the left, a tripwire laser shooting above the floor. Toss some bugbait to take out the guard, and an unlucky antlion will also trigger the mine.

22. Keep left. Activate this switch to open the gate visible through the glass. You can spy the Overwatch battling antlions on the nearby monitor (inset). Remember to change channels by clicking on the screen.

23. Grab the supplies through the doorway (1) and from the crates (2) before hopping the railing.

24. Continue through the now-unlocked door straight ahead. Toss a grenade to topple the turret far-right.

25. Your pulse rifle's secondary fire does a bang-up job of toppling the turrets and vaporizing the several Overwatch troops beyond the force field.

26. Smash this grate to gain access to the air vent.

27. Crawl through, then pick up this spade with your gravity gun. Toss it into the fan to stop its rotation. Continue through the air vent beyond.

28. An Overwatch is on the other side. Use your gravity gun to throw that crate at him, then pull the plug on the force field so your antlions can join you. Keep right to find supply crates, then load the next area.

29. Bugbait should be your weapon of choice here, both to take out the claymores and the many Overwatch. This area is cool because you can hear Dr. Breen's ramblings over the troops' radios.

30. Keep right and come to this hallway. Toss a couple barrels (or a flammable one if you're adventurous) to topple the dual turrets.

31. Continue up these stairs, using your crowbar to help the antlions fend off a swarm of manhacks.

32. Be ready to toss a bugbait at this Overwatch waiting in ambush.

33. Likewise, toss some bugbait down this walkway then retreat. Your antlions will make short work of the Overwatch. More troops are waiting atop the stairs ahead.

34. Toss some bugbait through the fence door to have one trip the claymore, then proceed through. Note that I've just picked up grenades from a crate in this position.

35. Allow your antlions to clear a path to that guard station ahead. This level really lets your antlions shine.

36. Once inside, bust open a crate for supplies, then fill up on health and energy at the stations on the right. Listen to Dr. Breen berate the Overwatch for their inability to capture a hapless physicist - you. Flip through the channels on the monitor to witness some grizzly experiments, then flip the red switch ahead to open the locked gate in the hall.

37. Continue straight ahead to load the next area, again letting your antlions blaze the trail.

38. Inside this office is more TV, a health station and a supply crate.

39. This is getting a bit repetitive. Have your antlions defeat these Overwatch, then toss a grenade down the left corridor to topple a pair of automated turrets.

40. Time for a rather sizeable battle. Toss bugbait at the three indicated spots then take cover. Re-apply bugbait as needed. Be sure not to trip the claymore mine blocking the office behind you.

41. Help out your antlions by toppling this turret with a grenade. Your path forward is up those stairs, but first take a brief diversion through the doorway on the right.

42. Grab supplies from the crates before entering the corridor. This is one nasty laundry room.

43. Find this nice stash of goodies in here, but don't dawdle. A toxic zombie is hot on your tail.

44. Turn around and wait in ambush, your shotgun at the ready.

45. Now you can continue up the stairs to this next engagement. Again, toss some bugbait to clear out the overwatch and to trip the claymore. Beware of turrets on the right.

46. Again, toss a grenade to topple the turrets. Note the supply crate on the left, holding secondary-fire ammo for your pulse rifle. Does that stuff have a name?

47. Open the indicated door and toss in more bugbait.

48. Several turrets are in this next room, and the shimmer of a gas leak is just visible ahead. Grenades will cause the gas to light, so quickly sprint within gravity gun range of the turrets and tip them over.

49. Use your antlions to trip this claymore, and topple another turret to the left.

50. Briefly pop through the doorway on the left, toss some bugbait to the Overwatch at the top of the stairs, then retreat until safe.

51. Break open the supplies and fill up on health. An energy station is on the opposite wall.

52. Crouch-jump through this broken window to the room below.

53. This battle between Overwatch troops and an antlion bull ensues. Don't worry about the Overwatch. Instead, pummel the antlion with rockets.

54. Once the Overwatch are gone and the antlion turns to you, keep firing with your pulse rifle (start with secondary fire) and the bull comes down shortly. Use your gravity gun to clear the doorway marked C3 ahead.

55. Ammunition is found in these supply crates.

56. Take out this zombie below the stairs.

57. Avoid the giant crushing things while moving debris with your gravity gun to make some makeshift stairs. Continue through the hole ahead to finish the level.