Chapter: Direct Intervention
Fun fact: this is the map with the 4th highest player casualty count in Episode One, according to Steam statistics
Finally you reach the Citadel's energy core, which you have to stabilise.
A welcoming party consisting of a handful of elite combine soldiers greets you. Quickly dispose of them, and Alyx will start fiddling with the computer console with the flashing alarms...uh oh...flashing alarms, that's never a good sign!
She opens up a door and calls a lift down to the actual core room. You're on your own from now on, buddy!
Note: if you're just playing through the game and came here because you're stuck, carry on reading. If you want to get the "Pacifist" achievement, please direct your attention here.
Grab an energy orb from one of the numerous streams around the core room and snipe both stalkers in view: directly in front of you on the far side of the room, and on the left side. If you fail to do so, they will destroy the energy balls you set in sockets, preventing you from progressing any further.
As you're probably accustomed to by now, cast energy orbs into receptacles, one directly above your head upon entering the room, and one on the far side, where the first stalkers you killed were standing.
Cross the bridge that you activated. From the other side, you have a clear shot at a 3rd energy orb socket, located above where the second stalker was standing. Cross the next bridge to where you just fired the orb, enter a small glass-walled room and press the big red glowing red button (don't you just love glowing red buttons?). The bridges will realign, giving you access to the central spire. Climb the ladder in the centre of the room; be wary of the energy beam.
When the ladder ends, move backwards over a short energy bridge, onto a ledge and go through an open doorway where combine troopers will be waiting for you. A few more will arrive from a small ledge above the small room, but you shouldn't have any trouble dispatching them.
This small room features another energy ball socket, so simply walk back to the main core room, use your gravity powers, and quickly walk back to the side room to throw the ball into place, activating a large generator to stabilise the failing core and opening a door to your next ordeal.
You will be attacked by soldiers from the doorway and manhacks from behind, but neither should pose much of a threat against the awesome blue gravity gun, especially when wielded by Crowbar Jesus.
The adjacent room is being traversed by energy orbs; enter it by removing the small door leading to it. You must make your way to the opening near the source of the energy orbs, and in order to manage that, you can try running and slaloming your way through them; jumping over the lower-flying ones; shooting the orbs with the Gravity Gun's primary fire; or you can use an object, like the tunnel's door as a shield.
Any way you wish to do it, exit the energy orb tunnel at the other end, climb the blue-lit room, and jump through the side of it, following the red pipes, to find another room full of enemies. Send them back to their maker (I wonder what the stance on theology is for trans-human constructs...), remove the energy ball from the white localised orb retention field, and enter the main core room.
Note: this room is probably the best place in the game to earn the "Conservationist" achievement.
Cross the bridge to the centre core, observe its pulsations and, at the opportune moment, run across the partially-melted ring to the other bridge.
Enter another generator room, very similar to the one you were in earlier, clearing it of combine troopers. This time, you don't need to run all the way to the core room to get an energy ball, you can just pick one up from the localised ball containment field in the generator room.
Exit the room through the newly-opened door and step on yet another lift. Fight your way through a handful of enemies, and find your way back into the core room proper, just above the core itself. Jump onto one of the 4 spinning metal thingamajigs rotating over the core, be careful with the attacking manhacks though, and jump off again as soon as you see a balcony-like structure, about 3/4s of a full rotation around the room (Alternatively, you can run near the axis of rotation and jump counter-clockwise on the rotating blades in order to reach the exit faster).
Fighting your way forward, follow the red lighting and a few turns later, you'll be back in a somewhat derelict energy orb tunnel; however, this time it's harder to find something to deviate the orbs with, although you can try to dislodge a large metal plate from the fire a few metres before. Dodging the orbs is much easier this time around, since they only glide on ground level; so slaloming, jumping and shooting them with the Gravity Gun is easier than before.
For the third time, you arrive at a generator room defended by combine soldiers. This time, you have an infinite supply of energy balls at your immediate disposal from the tunnel you just traversed, so fire away merrily through the tunnel's opening until the room is silent.
If you're lucky, your wild firing will have lodged a ball into its socket in the generator room, if not, grab a new ball from the tunnel and place it appropriately (you can pick up the orbs from the tunnel through a broken window). But what is this? ****** place is falling apart! Climb down to where the generator rose to find 3 empty localised orb retention fields and a broken ceiling with pipes hanging down from it, giving you access to the orb tunnel. Place an orb in each of the 2 rightmost receptacles, grab a 3rd one, walk back to the generator's lift from whence you entered and throw it into the 3rd field, making the lift rise back up.
Finally, activate the generator by putting a final energy orb into the socket.
At long last, the Citadel's core is restored to a more stable state *phew*. Sadly, the resulting energy surge downgrades your Gravity Gun to its normal orange state.
Exit the room through the door to find an exuberant Alyx, follow her and she recovers some data which will be instrumental in Gordon's and Alyx's future adventures, as well as a transmission from Dr. Judith Mossman. After this short pause, an alarm will sound, so keep following Alyx and get a lift out of there.
Fun fact: Beneath their suits, combine soldiers are fairly similar to the poor stalkers: humans combined with alien technology and several organs removed or replaced, hence the name "Combine"
Run through the corridor and don't bother trying to fight the combine soldiers, as your orange gravity gun is of no use without proper ammunition. Speaking of ammunition, as you arrive in a room Alyx locks behind you, two roller-mines will attack you. Hold them up to Alyx one at a time with your gravity gun so she can convert them to your side.
When the next door opens, let the roller-mines do their job of clearing the room of all hostiles and enjoy the show safely from behind the unbreakable glass.
When the coast is clear, Alyx will run through the room and board a train, heal up if you need to, then follow her in.
The train rolls on for a bit and eventually crashes. Make your way through the collapsed train and free Alyx using your Gravity Gun.
Exit the train and crouch through to finally exit the citadel.
Welcome to Half-Life Fallout - Gordon Freeman's source for everything on Half-Life and Valve.