Fun fact: at times, when everything is dark and still, Alyx will creep up behind you and make a zombie-like noise to freak you out. Probably a way to make up for some deep-seated mental scarring
Let Alyx take a breather from that traumatic experience on the train, and then move on.
Ooooh, zombies! Enjoy the tunnel while it's lit, and throw whatever you can find at the zombies and headcrabs, cinder blocks are surprisingly effective.
Navigate through the short tunnel, enter a dark hallway at the end of which you'll find a locked door and an air-vent, hidden behind a barrel on the right side. Ah! Air-ducts, where Gordon can feel right at home.
Keep your flashlight on and don't get tricked by the alluring medpack and flare directly in front of you, for there is a bottomless pit right in front of them. Gravity-grab the two items and turn left. Climb the ladder, drop into a room a few metres later, and proceed to kill the headcrab in there with a few well-placed Gravity Gun primary fire blasts.
You'll find a pistol, a shotgun, some ammo and a flare inside a cage on your right-hand side. Slide them through the hole in the cage's top corner and shoot the padlock on the chain-link fence door on the other side of the room. Find an electrical panel and activate it to turn on the lights and open the door for Alyx.
Switch to your gravity gun and enter the next slice of the car tunnel by clearing away the yellow car blocking your way. Enter and immediately go left to find more shotgun shells and suit energy.
Now in this part, you'll have very little to do in terms of actual fighting since you're still low on ammo. You're mostly going to illuminate Alyx's path with your flashlight in order to help her target enemies in the dark. But don't refrain from throwing heavy, explosive, and/or flammable objects around to help her.
Carry on through the tunnel until you come to a crashed Combine troop train, where you are introduced to a new type of enemy: the zombine...a zombie...combine...get it? So watch how the zombine behind the pane of glass acts and learn its weaknesses for future encounters.
Enter the room the zombine exploded in, exit the train on the right and move on into more darkness, fighting your way through all kinds of Xen creatures and derivatives. If zombines go suicide-bomber on you, use the gravity gun to remove the grenade and use it for your own destructive means.
20-30 metres later, behind a crashed blue and grey truck is a doorway on the left side of the tunnel (not the locked door with a cadaver in front of it), enter it and restore power to the area by activating an electric panel, like before. Fight some newly spawned zombies and backtrack to the previously mentioned locked door.
Get up two sets of stairs, and as you reach the top, a poison headcrab will try to ambush you from the left. But of course, reading this guide, you can see into the future and anticipate any surprises. Do you feel like a god yet? Either way, shoot it with your 12 gauge and grab the supplies on the top of the stairs. Additional health and suit energy can be found in the nearby locker, but dispose of two more poison headcrabs first.
Arriving at a pit in the ground that spawns infinite numbers of antlions, you can try Gravity-blasting them to flip them over and let Alyx finish them off, but eventually, you're going to have to do something about that spawning pit if you ever want to get out of City 17.
Hmmm...a wrecked car... Let's push it into the pit. Hey! That works! The antlions can't burrow their way up anymore. Good job, pat yourself on the back and start turning that heavy wheel in order to open the garage door.
Oh crap, Poison zombies coming through the door! Stop for a second to fight them off and resume raising the door.
Note: Now that you have access to cars and antlions, you can start working on the "Car Crusher" achievement, but trying to farm for the achievement in this spot is not recommended.
Two regular headcrabs and some supplies later, you stumble upon a chain-link fence leading to a dark hallway full of barnacles, as a legion of zombies starts shuffling towards you through the hallway. Don't panic, take your time. You can either shine your flashlight on the barnacles and have Alyx take them out for you, clearing a path; or you can trick the barnacles into capturing inanimate objects, like these handy barrels on your left, using that as a distraction to plow on through the hallway; or you can simply sit back, make some popcorn ,and watch zombies being eaten by barnacles.
When the coast is relatively clear, proceed forward. There are a few explosive barrels at the far-end of the hallway, which you can put to good use against the zombies. Take one of the regular barrels from earlier with you for good measure, you can never have too much ammunition.
Proceed through the double doors and up the metal staircase.
Fun fact: Alyx's unmistakable attraction for Gordon's mute manliness really starts showing in this map, when she asks if there's "room for two" in his HEV suit
Moving upwards, you find yourself besides a large multi-storey parking hall. Learn your surroundings from behind the fence, let Alyx take out the zombines if you want to conserve ammo, and pass to the other side of the fence through the opening far in front of you. If at any time you need healing, there are supplies in a white-lit room on the far side of the ground floor, along with a jerrycan.
Remember how to block off antlion spawning pits with car wrecks? This skill will become very useful here. There are three pits on the ground floor, but only one car. Move it over one of the pits and walk up the broken concrete beam to the next storey.
Move through the doorway into the room illuminated by yellow fire, directly on front of you; pick up supplies from the crate, and push the car down to block off another pit. Remember to Gravity-blast the antlions so Alyx can take them out for you.
Two spawning pits blocked, one to go.
The 3rd car is a bit trickier to get to. Move on back up, across the second storey and up one more beam to the top storey. Move across the room again by either walking across the beam or walking along the wall, which is much easier. Ignore the wheel-enabled doorway for now and drop down to where the white car lies, but only after readying your shotgun for the Zombine stupidly trying to trick you by laying still.
Turn the wheel on the right side of the grate-door until the gap is wide enough to get the white car through, use your Gravity Gun to dump the wreck downstairs, and, at last, block the final antlion spawning pit.
Make your way back up one last time where finally, you can turn the wheel opening the exit door from this nightmarish parking structure.
Climb all the way down the metal staircase into a shallow pool of water, quietly mock the zombie torso waiting for you here and grab some supplies from the crate under the steps. Climb one level back up and pass through the door.
Arriving in a mildly flooded room, use your shotgun, Gravity Gun and explosive barrels to battle off the undead human combine.
Moving along, you come upon a large dark room with a lift. Pile up as much rubble, barrels, explosive canisters and ammunition as you can near the lift, as you'll have to survive a relatively massive battle against a large force comprised of zombies, fast zombies and zombines.
Soon, the lights will go out, so you have to find an electrical panel in order to activate the lift.
Walk directly away from the lift to find a bluish-grey door blocked by a wooden plank. Lift the plank away with your Gravity Gun, open the door and quickly back away from the zombine that ambushes you in the room.
After disposing of it, get in that room and activate the electrical panel on the right hand side. Rush back to the lift and be ready to hold your ground. Fight off the enemies using your stockpile and remember to illuminate targets with your flashlight for Alyx as well. Avoid sprinting, as you'll need your suit energy to power your flashlight. When you run out of flashlight power, use flares or light zombies on fire (by holding a flare with your gravity gun and walking it into them, or tossing flare and flamable canisters) in order to maintain visibility.
A couple of minutes of fighting for dear life later, the lift arrives; immediately get on board and enjoy a few seconds of well earned respite.
Welcome to Half-Life Fallout - Gordon Freeman's source for everything on Half-Life and Valve.