After some encouraging words from Barney, you drop down into a long, dark tunnel. At the end of the tunnel, you'll find a ledge with a view of the Combine's foreboding superstructure on City 17. This foreign and alien-looking building also serves as the headquarters of Dr. Breen's administration. On this ledge, you'll also find two Medkits and the charred remains of some unlucky bastard who seemed to make it this far before mutating into a crispy meat twinkie.

As you look over the edge of the tunnel, drop onto the rock directly below and to the left of you. From here, it's a hop, skip, and some hair-raising jumps across the other rocks as you make your way towards the creepy alien HQ. A headcrab will make a leap at your head and miss by a mile, falling into the abyss. Once you're at the last rock, walk out onto the broken pipe and jump onto the piston structure while it's at the top. Then quickly scramble to your right and onto the narrow ridge encircling the structure.

Make your way up and around the structure, being careful to avoid getting pancaked by the other pistons, and drop down into the trough. You'll see the first bits of the Combine's intricated rail people-mover system, which transports members of the Combine's subjugate slave races to different processing points.

From here, make your way across the dark, winding path until you come to a place where you must jump to a lower level. Make the jump, then drop down further to another lower ledge. From here you'll be able to see the platform that's been making the zapping noise that's been growing steadily louder. Presumably, it's an energy beam that disintegrates all "unsuitable" Combine slave candidates. Keep on going.

You'll come across a ledge where two lines of people-movers are loaded. Don't jump into a mover just yet; if you jump into the first line you come across, you'll be powerless to move as you're directed right into the zapping machine you saw earlier and obliterated. Instead, wait until the people-mover has left and cross to the other side of the ledge. Get into a mover from this side of the ledge and you'll be taken on a trip deep into the heart of the Combine.

You'll have nothing to do but enjoy the ride, as you pass the deepest inner workings of the Combine. Alien slaves are hard at work on the Combine vehicle production lines, all the while under the careful eye of Combine soldiers. A group of Striders will pass below you on their way out to City 17. You'll get a glance at the Combine's railway shipping system, which brings raw materials in from the outside to keep the production lines going 24/7.

After the scenic trip, you'll be dropped into a confiscation energy field and all of your weapons will be taken from you by the field and disintegrated... all except for the gravity gun. An alarm sounds as the field senses the strange weapon, then the field shuts off. You're now able to pick up the gravity gun, although it seems the energy field has unintentionally supercharged it and it now glows blue instead of orange, but suddenly you've got Combine troops coming down the hallway at you. You'll find that you're now able to pick up Combine soldiers with the right click and punt them with a left click, or just left click to punt on contact.

Make quick work of the incoming troops, then head down the hallway. The HEV suit stations in here are all juiced up like Barry Bonds, so take advantage of the one nearby to load up on health and 200 points of armor. Nail some more troops with your new fun gun, and make your way to a second set of computer screens, with Dr. Breen sermonizing some nonsense again.

Further on, you'll come across an energy field. Whatever you do, don't walk into the core, or bad things will happen. Instead, fling combine soldiers in and watch them move in slow motion towards the beam before making contact with the core and disintegrating. Take the path that forks right for an elevator.

The elevator will take you up and behind a force field, with two Combine troops firing at you from a distance and standing next to an energy field with bright white energy spheres moving up and down. Grab the lone sphere on your left with your grav gun to drop the field, then fling it at the troops. You might notice that these energy spheres are very similar to the secondary fire grenades on the Overwatch Pulse Rifle, and the spheres are gonna be around a lot from here on out. This is, of course, a good thing.

Take a right and load up on health and armor at the station before proceeding and coming to another energy field. Grab the energy sphere fueling this field from the nearby machine and drop the force field, then prepare for an onslaught of troops from two directions. Keep in mind you have a well of energy sphere streams in front of you, and put them to good use. Once you've dispatched of the troops here, move on.

You'll come across an open area with a switch in the middle while Dr. Breen continues ranting at you. You'll need to flip the switch right away to get the elevator moving while you take care of the waves of troops, scanners, and manhacks about to attack. There will also be a couple of troops taking potshots from a ledge above you, grab them with your grav gun and bring them down to your level.

While the elevator is coming down, keep an eye on your health and reload at the HEV station, if necessary. Once the elevator platform has come down, just step onto the central platform to start moving up. While on the platform, you'll continuously be attacked as you make your way up. Do your best to dispatch troops as quickly and efficiently as possible and make use of the energy sphere streams around the elevator.

At the top, take care of the Combine troops and then remove the energy sphere to deactivate the force field around the elevator. Load up at the HEV station and continue. You'll come to another energy field, and more troops. Grab the troops and fling them into the energy field core for some quick Combine S'Mores, then take the right fork as before. Fill up at the HEV station, then continue until you're looking down an enormous hallway, a giant hangar of sorts. Breen will continue yapping at you from a nearby terminal, listen if you wish.

Drop down into the hallway and be prepared for a rush of troops from a distance. They have the advantage of range here, so you'll want to make a beeline for them to decrease the distance and put your grav gun into use. If you need to recover your sprint meter, duck behind a people-mover container for a breather.

Once you've dispatched of the initial wave, further down the hallway you'll see giant doors open up and a Strider comes out. Again, the thing to do here is to run straight at the problem: you'll need to get past the Strider to get at the energy sphere stream behind it, and a quick rush also ensures that the Strider can't fire at you because you're right underneath it.

After you've removed the energy spheres from the force field behind the Strider, you can either stick around and kill off the Strider using the other energy sphere streams in the hangar, or just jump down off the ledge to safety.

Either way, once you jump down you'll be safely out of the Strider's way (if you didn't kill it) and there will be a HEV station. Load up, then head on down the hallway to see a people-mover loading point. Nothing to do here but jump on. After observing a creepy alien in another people-mover, then you'll pass through a red beam which will detect you and send you off on a secondary rail skywards through the structure's spire towards Dr. Breen's office.

You're stuck like a rat in a cage, your future uncertain, with nothing to do except enjoy the view of City 17 and wait for Chapter XIV: Dark Energy to unfold.