Both the starting area and exit area are unique for the alien. When you start, crawl out of the water and up the ledge. Turn left and drop down the hole. Run to the left and take the first left. Drop down and go to the generator. Destroy it with your tail. This will give you some time in the darkness, then in little light. Easier to kill the Marines like this. Run through the exit and kill the civilians. Go straight ahead, then turn left. Go right into the turning corridor and follow it left. Open the gate and turn left. Climb up the walls here and go through the corridor. Take the first right and through the door. Go along the right wall and into the next corridor. Go through the computer room, through the door, and drop down to your left. Go through the tunnel to your left and out the door. Jump down and over to the door on the far right, under the stairs. Take out the turret there. Turn left, then right, and through the door. You're now at where the marine started, but you have a little further to go. On left, on the ceiling, is an air duct. Climb up through it. Go through the series of halls on the left, away from the turret, and follow them to the door at the end. Go through the door. On the ceiling on the left is another air vent. Go through it. Climb up the shaft to end the level