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"You don't know what you'll unleash! You could bring down this whole Citadel! Think, man, think - of the people below!"
Wallace Breen to Gordon Freeman

The Citadel is a gigantic Combine megastructure located in the heart of City 17, which serves as the main base of operations and primary headquarters of the Overwatch, from which they govern the Earth. Easily the most striking symbol of the Combine's rule, the Citadel looms ominously above the city as a constant reminder of their presence, and serves as the personal residence of Earth's appointed Administrator, Dr. Wallace Breen.


Citadel down

The Citadel before its complete destruction.

The Citadel is located at the epicenter of City 17, and resembles an enormous metallic skyscraper thousands of feet tall. A true monolith, it is a fortress that rises both high into the sky and extends a great distance underground as well. It is a triangular structure when viewed from above, and is crowned by a Dark Fusion Reactor, with its associated portal acting as the primary means of communication with off-world Combine forces. The Citadel's massive interior level is capable of housing a massive army of Overwatch Soldiers, with the building itself being heavily mechanized, and entire sections of it can move to allow for mass deployment of airborne Combine units such as City Scanners, Hunter-Choppers and Combine Gunships.

The Citadel is completely surrounded by enormous Smart Barriers which protect the structure from would-be attackers attempting to breach the walls. Attached to the exterior walls of the Citadel, within a huge chasm surrounded by broken sewer pipes and destroyed vehicles, are large pieces of machinery that dig the ground, possibly expanding the underground area to allow for further construction. These machines are also theorized to function as giant Restrictors to repel Antlions, though such theory is unlikely as the vibrations could not cover the entire city. The game files also mention the machines as "Diggers" instead of thumpers.

The Citadel has two known "states" or configurations that the structure takes depending on specific situations. The first of these is the dormant configuration, which was most likely maintained throughout the majority of the Combine's rule, when no serious threats were posed. This configuration is characterised by a single set of rib-like structures exposed within the rightmost half of the Citadel, with an outer metal plating retracted slightly above an inner plating. However, shortly after Gordon Freeman is transported inside Breen's office in the Citadel during the malfunctioning of Dr. Kleiner's teleporter, Breen puts the Citadel on alert, causing the structure to change to its alerted state. Barney Calhoun notes that this is a rare occasion, stating that he's "never seen it lit up like that." The alert configuration involves the inner plating retracting upwards to reveal a total of four sets of "ribs", as seen at the start of Route Kanal whereupon the reconfiguration has fully completed.

During this transformation, a loud alarm is heard resonating throughout the city, possibly emanating from the Citadel itself, and Scanners are seen rapidly deploying from the structure's midsection to hunt for Freeman. The Citadel remains in this configuration for the remainder of the game, only changing again during Episode One after Freeman sabotages the reactor, with the plating descending once more to the "all clear" state, covering the ribs. 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 games.

The book Raising the Bar mentions that entire blocks from City 17 vanished into thin air as the Combine delivered the Citadel to Earth. However, this was likely changed during development as the game shows no signs of such an event. Half-Life: Alyx shows the Citadel under construction, confirming it was built by the Combine after the Seven Hour War. Based on a drawing in Alyx's apartment, only the Citadel's Core was teleported to Earth as single spire roughly the same height as the completed Citadel, with the rest of the architecture constructed around it.

The existence of more Citadels located around the world is mentioned in-game by Dr. Kleiner and is also mentioned by Eli in The Final Hours of Half-Life: Alyx. The number of Citadels on Earth still remains unknown, as do whether they all bear the same height and general appearance.

Lower levels[]

Dark Fusion Reactor 1

The Citadel's Core, exceedingly close to collapse.

In the first few chapters of Half-Life 2: Episode One, the player is given a first-hand look at much of the lower levels of the Citadel, mostly looking very much the same as the rest of of the structure, with no war factories or synth manufacturing plants located in this area. During this point in the game, the Citadel is observed to be extremely unstable due to Freeman's sabotage, with an orange glow replacing the cool blue it used to be when fully functional. Fires have spread across the structure and various pieces of debris from what's left of the upper levels is seen falling down to the lower levels.

An emergency evacuation and transport chamber for Advisors is located in the lower levels of the Citadel. The room is significantly different from the rest of the Citadel, giving off an eerie red glow from hundreds of bright lights mounted on the walls, giving it a decidedly more sinister appearance. The Advisor Pods are located under the glass floor of the room and, when preparing to evacuate, they rise up and move through a tube before positioning themselves in a chute and moving upwards. It is likely that many more chambers are present throughout the Citadel.

Toward the very bottom of the Citadel is its highly radioactive Core, which is only accessible via a lift that descends for several dozen floors. The Core chamber is protected by large blast doors carrying an insignia appearing as a stylized version of the international symbol for radioactivity. A large control room is situated overlooking the Core chamber, similar in design to the control room overlooking the Dark Fusion reactor at the opposite end of the structure. Three rooms are located across the chamber which are specifically designed to help stabilize 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 to the Core chamber is a train station, where it is likely that Razor Train cargo is emptied. This area of the Citadel lies approximately one kilometre below the surface, and the connected train tunnel network extends throughout City 17 and beyond.

Central levels[]

Razor train

Razor Train seen inside the Citadel.

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. An abundance of Combine technology is 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 resembling arteries that transport unknown fluids. The entire building is constructed from the same blackish blue metal material that all Combine structures are made from. No attempt is made at all to provide personnel with any form of warmth or comfort. Everything is sharp, cold, and soulless, mirroring that of the Combine's rule. No regard is placed at all to the covering of the piping and electronics. The metal is unfinished and oxidizing. The human features that are present however, such as walkways, stairs and ladders, seem to just be an afterthought, as if the usability for humans was not a priority. All invading "visitors" have any unregistered weapons stripped away and disintegrated using a Confiscation Field, presumably to avoid precisely the situation which occurs at the end of Half-Life 2 and recurs at the beginning of Episode One - ironically, because of the field.

There are railway systems running throughout the Citadel that are used by Razor Trains to ship and deliver cargo. Much of this cargo consists of 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 are 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 and delivered away. 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[]

Half-Life 2 Dr Breen Office

The interior of Doctor Breen's office at an upper level of the Citadel, as Gordon Freeman is transferred in.

Before reaching the very top of the citadel, Freeman passes through an area of the where enormous segments of the Citadel's exterior are open to allow Dropships and Gunships to quickly enter and exit the interior.

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-though the walls do not match his "studio" backdrop, suggesting there is a separate room for that purpose. 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, tiled floors, and busts of himself, much like the generator rooms in the Overwatch nexus. His office also contains a large set of monitors, which act as his personal computer, and from which he can communicate with contacts such as the Advisors. 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 or a separate door from the prisoner entrance. It can be assumed that Breen lives in the Citadel as well as works in it, presumably never leaving.

The very top of the Citadel contains the Combine's dark fusion reactor, directly connected to Breen's office, which is used to power the Combine's tunneling entanglement device, allowing for inter-universal translocation. At the end of Half-Life 2, Gordon manages to destabilize and destroy the dark fusion reactor using Energy Balls, destabilizing the entire Citadel.



The Citadel during its construction as shown in the Half-Life: Alyx trailer.

The Citadel is under construction at the time of Half-Life: Alyx, five years before the events of Half-Life 2, or roughly fifteen years after the Seven Hour War. This suggests that either the Citadel took several years to build or that construction began relatively late into the Combine occupation, or perhaps both. Another possible explanation is that the Combine had to wait for the cessation of the Portal Storms before they could safely start construction.

After the initial invasion, the Combine delivered the Citadel's Core to Earth through means of teleportation, itself contained within a tall spire structure roughly the height of the completed Citadel which was embedded into the ground; the rest of the Citadel was then built around it from deep underground all the way to the top. The Citadel construction consists of gathered resources from Earth, which were collected by the Combine and converted to their bluish material. [3]

Attached to the construction areas all over the Citadel are several sets of slack grouped cables, with substation structures on the ground providing support to them, most likely providing energy to the Citadel from an external source, as the Core was non-functional and suffering blackouts by this time. The location of the core on Alyx's whiteboard suggests it was moved to the underground levels later on. [4]

The group of cables seem to be connected to elevator like structures, called 'scaffolds' by Alyx, which are presumably tasked with automated construction or mass transport of materials. Sets of smaller cables resemble conventional power lines, suggesting the structures function in a similar way to substations. These cables were connected to the City 17 rooftops by construction striders.


Episode1 Last Chapter

The Citadel shortly before its destruction in Episode One.

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 core's inevitable meltdown. By this time, all the Combine forces were being evacuated from City 17 with only some remaining to stall the Resistance-led evacuation attempt. As a result, the Citadel is barely occupied compared to its prior state.

The Citadel's interior now glows orange, in stark contrast to the cold blue it used to be when functional, with bridges falling apart and debris continuously raining down from the top of the tower. From the outside, the Citadel's peak has been sheared off from the explosion, and a glowing red spacetime anomaly, likely a remnant of the previous connection to the Overworld, looms above it surrounded by swirling black and red clouds with occasional green lightning bolts. The power lines connecting the Citadel to City 17 are no longer present.

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 distress call off-world to the rest of the Combine, causing an immense Dark Energy Flare that destroys the entire Citadel along with the surrounding City 17. Dr. Kleiner notes in one of his messages to the Resistance that the destabilization of the City 17 Citadel has also "forced a damper on the entire network of linked Citadel reactors'", causing the other Citadels across the planet to shut down and sever the Combine's connection to the Overworld. This implies that the Citadel served as the primary 'link in the chain', from which most if not all Combine inter-universal portal technology was dependent upon, further demonstrating its importance to the Combine and exposing a vulnerability in their rule.

City 17

The Citadel in its complete ruin.

In Half-Life 2: Episode Two, the Citadel still dominates the now destroyed City 17 even in its ruin. A blue tornado-like vortex stretches high into the clouds, created from the raw discharge of the Core's meltdown now focused into a Superportal. The Superportal looms over where the Citadel once stood, slowly gathering strength until its large enough to fully open the gateway between worlds and allow the Combine to re-invade. At the end of the game after the Superportal is neutralized, the Citadel's collapsed remains are all that is left of the once tall and overpowering structure.

Behind the scenes[]

The Citadel went through various design stages during the game's development, as seen in several concept art images and the WC mappack. It started as a brush until becoming the model seen in Half-Life 2.


  • Some designs were also taken from the Half-Life chapter Interloper, especially in the dark fusion reactor.
  • The current design is reminiscent of the Bayonets Memorial in Zelenograd, Russia, as well as a monument in Aberystwyth University in Wales.
  • The Citadel was originally to feature a garbage incinerator, although it is not clear in what version of the Citadel it was to be located.[2]
  • The final showdown dialogue between Breen, Mossman, Eli, and Alyx was originally to be much different and much longer.[5]
  • The "Kleinercast" featured in Episode One implies that the Combine built multiple Citadels around the world, and that each one is linked through their dark fusion reactors.
  • In the files for Episode Two there is an unused animation of the Citadel exploding, with debris flying towards the player.[6]
  • A reddit user known as "rscreamroad" calculated what the real-world dimensions of the Citadel would be by studying the game files and other production information, concluding that it is 27,580 feet (8,406 meters) tall, 2,820 feet (860 meters) wide, and extends 750 feet (230 meters) underground. For comparison, Mount Everest is 29,032 feet (8,849 meters) tall.[7][8]


Concept art[]


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