Citadel

Physical construction and overview


Citadels are the giant structures from which the Combine govern the Earth. One is located in City 17, and resembles a massive, metallic and largely featureless skyscraper, rising several kilometers into the sky from ground level, and extending some distance underground as well. The City 17 Citadel is heavily mechanized, and entire sections of it can move to allow for mass deployment of airborne Combine units such as scanners or hunter-choppers. It appears to either provide power to City 17 or drain power from it, as power lines can be seen extending out of it. These power lines then join on to regular transmission towers and poles, connecting it to the rest of the city.

It should be noted that the Citadels (at least that in City 17) have two known "states," or configurations that the tower takes depending on specific situations. The first of these is what is most likely the dormant configuration, which is probably maintained during the majority of life in the city, when no serious threats are posed to the Combine (the structure also takes this shape during Half-Life 2: Episode One). This configuration is signified by a single set of rib-like structures exposed within the right half of the Citadel, with an outer metal plating retracted slightly above the inner plating. However, shortly after the player is transported inside Doctor Breen's office in the City 17 Citadel during the malfunctioning of Dr. Kleiner's teleporter, the Citadel enters an alerted state. The Citadel can be seen changing its configuration while Barney Calhoun gives Gordon a crowbar. Barney says that this is a rare occasion, stating "I've never seen it lit up like that." The inner plating retracts upwards, exposing a total of four sets of "ribs," while a loud alarm sounds throughout City 17. A small piece of plating barely visible on the left of the Citadel also descends partly down the tower, and the Citadel remains in this configuration for the remainder of the game. It can be theorized that there are other configurations the Citadel may take in different situations, but these two are the only configurations depicted in the game.

As mentioned in the Raising the Bar book, entire blocks from City 17 vanished into thin air as the Combine delivered the Citadel to Earth. There is a slight mention of this event in Dr. Breen's first speech in Half-Life 2, during which he states that the Citadel was "thoughtfully provided by Our Benefactors." The Citadel is embedded in a huge chasm with antiquated aqueducts and newer sewer pipes with broken ends extending in from the walls of the pit and cars, now scrap metal, lying at the bottom of the pit: making it seem as if the Citadel plummeted into the ground forcefully from a high distance, and is completely surrounded by enormous mobile walls which chew up material around the Citadel and prevent access from the streets. Attached to the base of the Citadel are other machines that pound the ground. These are theorized to be used for destroying the city, acting as giant thumpers to repel Antlions, or gathering raw materials from the city's structures, but their true purpose remains unknown. It should be noted that the number of Citadels in Earth remains unknown in game. It also is clearly visible that there are numerous other Citadels outside Earth, as seen through an off-world portal which Dr. Breen intends to enter during the final moments of Half-Life 2.

Lower levels
In the first few chapters of Half-Life 2: Episode One, the player is given a firsthand look at much of the lower levels of the Citadel. Whilst mostly looking very much the same as the rest of the Citadel, no war factories or synth manufacturing plants are located in this area of the structure. During this point in the game the Citadel can be seen as extremely unstable, with an orange glow sweeping through the structure in contrast to the blue it used to be when functional. Fires have spread across the structure and much debris from what's left of the upper levels is constantly falling down to the lower levels.

An emergency evacuation chamber is located in the lower levels of the Citadel. The room is significantly different from that of the rest of the Citadel, giving off an eerie red glow in contrast to the blue and orange that characterises the Citadel at the time. The room seems to serve as an emergency evacuation chamber, allowing Combine Advisors to quickly escape in times where the Citadel is in peril. The Advisor escape pods are located under the glass floor of the room and when preparing to dock they rise up and move through a tube before positioning themselves in a chute and moving upwards.

Toward the very bottom of the Citadel is its core which is only accessible via a lift that descends several dozen floors. The core chamber is protected by large blast doors carrying an insignia with the appearance of a stylized version of the international symbol for radioactivity. A large control room is situated overlooking the core with a central access lift leading down to its chamber. Three rooms are located across the chamber which are specifically designed to help stabilise the core in the event that its energy output becomes unstable. The machines in these rooms form a threefold containment system that ensures that the intensity of the dark energy reaction within the core stays within safe limits. Connected onto the core itself is a train station where it's likely that Razor Train cargo is emptied. This point in the Citadel lies approximately one kilometer below the surface and extends throughout City 17 and beyond.

Central levels


In the final chapters of Half-Life 2, the player is given an insight into the interior of the Citadel, which contrasts its featureless exterior. Abundances of Combine technology are found within the Citadel, ranging from countless generators and force fields, to matter disintegration fields, to elevators and platforms that defy gravity, to giant tubes that transport unknown fluids. The architecture of the Combine has its greatest presence in the Citadel. The entire building is constructed from the same blackish blue metal which all Combine structures are made from. All "visitors" routinely have weapons stripped using a confiscation field, presumably to avoid precisely the situation which occurs at the end of Half-Life 2 and reoccurs at the beginning of Episode One – ironically, because of the presence of a confiscation field.

There are railway systems to and from, and even within the Citadel that are used by Razor Trains to ship and deliver cargo. Large amounts of the cargo are new Stalkers from Nova Prospekt. Stalkers are delivered in pods, and the Citadel includes a large transportation system for these pods. Tasks such as the operation of basic panels, or the repairing of damaged Combine equipment and units is done by Stalkers, under the constant supervision of Overwatch personnel or scanners.

The Citadel is presumed to include large war factories, where Combine equipment, technology and weapons, as well as Synths are manufactured. Two types of Synth that were never encountered elsewhere during the game, the crab synth and the mortar synth, can be seen being transported around on conveyor belts, indicating that they are possibly manufactured in the Citadel, although this could only be a means of moving the synths throughout the large facility. The Citadel also provides a central hub for Overwatch soldiers (including Elites), who are present in overwhelming numbers, and is the launching point for gunships.

Upper levels


On one of the very top levels of the Citadel is Doctor Breen's office, believed to be the location from which he transmits his Breencasts. Unlike the rest of the Citadel, Breen's office has a more classical, human look to it, although Combine architecture is still a very dominating feature in the room. His office includes various luxury items such as a wooden desk, mantle clock, leather seat, globe, expensive carpeting, and busts of himself. His office also contains a large set of monitors, from which he can communicate with contacts such as the Combine Advisor. Breen's office is connected to the prisoner transport system used by Gordon Freeman, and there are many other prisoner pods around his office. Otherwise, more welcome guests can access the office via an elevator connected to the dark fusion reactor. It can be assumed that Breen lives in the Citadel as well as works in it.

The very top of the Citadel contains the Combine's dark fusion reactor, which is used to power their tunnelling entanglement device and allow them to tunnel into Earth from their universe. At the end of Half-Life 2, Gordon manages to destabilize and destroy the dark fusion reactor using energy orbs.

Destruction


In Episode One, the aftermath of Gordon's assault on the Citadel is clearly visible. The overloading of the Dark Fusion reactor has downed power and destabilized the structure of the whole building, and Gordon must make his way to the core to re-engage the containment field and delay the cores inevitable meltdown. The Citadel is by this time barely occupied compared to its earlier state, presumably most of the soldiers and synths have been evacuated into City 17 to prevent the Citizens from escaping the blast. There are however, several soldiers left, and a few Stalkers for operating machinery. The Citadel's interior now glows orange, in stark contrast to the cold blue it used to be when functional, while bridges are falling apart and debris continuously falls from the top of the tower. From the outside, the Citadel's peak is surrounded by swirling black and red clouds with green lightning bolts emerging. The power lines connecting the Citadel to City 17 are no longer present, and the explosion of the Dark Fusion reactor has blown the top of the Citadel off. The climax of Episode One is witnessed by Gordon and Alyx, when the Citadel's core suffers the inevitable meltdown as it collects the colossal amount of energy needed to send a final data transmission off-world to the rest of the Combine empire.

Scenes from Half-Life 2: Episode Two show the remains of City 17 from a distance, with a smoking mass at the center where the Citadel once stood.