When you enter, kill the civilian and follow the vent around the corner. Either shoot the glass and then use the grappling hook to get through it, or shoot the hook to the ceiling above the glass and smash it with your Wristblades. When you get through, kill the humans at the top and then fall down. You can now either kill the aliens and then kill the marines, or leave them to themselves. When you're down, continue through the double doors and then two more sets of the same style. Behind the third door is a marine - kill him and then an alien will leap from the vent to attack you. Dispose of the bug and enter the vent he entered from, using the hook obviously. Follow it round to enter a room full of egg transports. Here there is a civilian and an alien. Dispose of both, then continue through the double doors to the right of you when you exited the vent. Go through yet another set of doors and then another. You're now in a lab. Here you'll meet an android and facehugger, and quite possibly an alien or two. When they're all dead, pull yourself up to the platform above you using the grappling hook, and go through to the next lab, where there is some field charge to your right. Ready your pistol and electrical view, then enter the next vent, where there's an alien. After killing it, fall down into the next room, where there are two humans. One side of the room is actually the entrance to the airlock, so pass through it, killing the alien in the second chamber. On the other side, kill the marine and pull yourself up into the vent. Follow it round, then go straight over the platform into the next one - there's nothing worth looking at down below. When you exit the vent and fall off the platform, you'll find yourself in a long, thin corridor, with two rooms connected to it. Dispose off all enemies and then move along to the viewing gallery, where you get to see several dumb-assed humans get their asses kicked by a xenoborg. When you're done, continue down the corridor to enter a toilet - where an alien will attack a civilian. When both are finished, pull yourself up to the platform above the door that you entered. Collect the field charge, then continue through the vent. Drop down on the other side, killing the humans and passing through the door. Kill the ones on this side too, then switch to electrical and select your disc. As soon as the doors open in the corner, target the xenoborg and let him have it. Retrieve your precious disc, then pull yourself up onto the platform and head into the vent. There's an alien to dispose of here, so do so, then fall down and kill the marine and the alien that arrives. Use the grappling hook to get up into the vent, then walk through into the temple and fall down to the bottom. Pull yourself up into the hole, then walk through. Dispose of the Praetorian to end the level