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The Citadel is located in City 17, and resembles a massive, metallic skyscraper, rising numerous kilometers into the sky from ground level, and extending some distance underground as well. It is a triangular structure when viewed from above, and was crowned by a Dark Energy Reactor. Its associated portal was the primary means of communication with off-world Combine forces. The Citadel is heavily mechanized, and entire sections of it can move to allow for mass deployment of airborne Combine units such as City Scanners or Hunter-Choppers. A large factory where Combine Gunships are repaired and augmented can be found in the Citadel, and a large flap on the citadel's exterior lifts up to allow the synths to exit the Citadel to continue the war against The Resistance.
The Citadel has two known "states," or configurations that the tower takes depending on specific situations. The first of these is what was most likely the dormant configuration, which was probably maintained during the whole life in the city, when no serious threats were posed to the Combine (the structure also takes this shape during Half-Life 2: Episode One). This configuration was 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 Gordon Freeman was transported inside Doctor 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. 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", at the start of Route Kanal, where the reconfiguration has fully completed. During this transformation a loud alarm resonates, echoing for several seconds each time it sounds, possibly emanating from the Citadel itself. A small piece of plating barely visible on the left (southern) side of the Citadel also descends partly down the tower, scanners rapidly deploy from around that area on the tower, and the Citadel remains in this configuration for the remainder of the game. The Citadel can also be seen from the rear near the first gate and "Water Hazard", showing a dull pair of metal strips separated by one vertical white beam. When Freeman sabotages the reactor, the plating descends to the "all clear" state and covers the ribs. The lower set of ribs descends along the tower and locks in place behind the lowest panel on the right-hand side of the structure. 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.
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. --The trailer for Half-Life: Alyx shows the Citadel under construction, confirming it was built by the Combine after the Seven Hour War. It should be noted, however, that the Core was teleported to Earth as a pinnacle structure about the size of the citadel into the ground, likely contained within the structure.
The Citadel is completely surrounded by enormous Smart Barriers which crush buildings around the Citadel and prevent access from the streets, protecting the structure and preventing rebels from advancing.
Attached to the exterior walls of the Citadel, in a huge chasm with broken sewer pipes and destroyed vehicles, are other machines that dig the ground, possibly expanding the underground area to allow further construction or creating exterior access to the lower levels. The chasm does not surround the Citadel, but instead surrounds only the machines, this implies the whole chasm was dug by them. These machines are theorized to be used as giant Restrictors to repel Antlions, though such theory is unlikely as the vibrations could not cover the entire city. The game files and the book also mention the machines as "Diggers" instead of thumpers, implying its digging nature.
The existence of more Citadels located around the world is mentioned in-game by Dr. Kleiner, however, the number of Citadels on Earth remains unknown, it is also unknown whether they all bear the same altitude and appearance.
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 extremely unstable, with an orange glow throughout 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 and transport chamber for Advisors 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 from hundreds of bright lights mounted on the walls in contrast to the blue and orange that characterizes the Citadel at the time. The room appears to be used as a transport chamber, but may also be used for evacuations, allowing Advisors to quickly escape in times where the Citadel is in peril. The Advisor 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. It is likely that many more chambers are present throughout the Citadel.
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, 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 onto the core itself is a train station where it is likely that Razor Train cargo is emptied. This point in the Citadel lies approximately one kilometer below the surface and the connected tunnel extends throughout City 17 and beyond.
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. No attempt is made at all to provide personnel with any form of warmth or soft aesthetic. Everything is sharp, cold, and unfeeling, 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 firsthand priority.
All invading "visitors" have weapons stripped 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 presence of a confiscation 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.
Before reaching the very top of the citadel, Freeman passes through many open areas of the citadel where enormous segments of the citadel exterior are open to allow Dropships and gunships to quickly exit and enter the citadel's 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, albeit aesthetic, 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 it, as the Advisors.
The very top of the Citadel contains the Combine's dark fusion reactor-that seems to be directly connected to Breen's office, which is used to power their tunneling 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 Balls.
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 be built or that construction began relatively late into the Combine's invasion, or perhaps both.
The initial form of the Citadel started when the Combine delivered the Core to earth, contained withing a spire structure that was embedded into the floor through the means of teleportation; the citadel was then built around it from underground to top. The Citadel construction consists of gathered resources from Earth, which were collected by the Combine and converted to their bluish material. 
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. Those were most likely providing energy to the Citadel from an external source, as the Core was nonfunctional and suffering blackouts by the time. The location of the core on Alyx's whiteboard suggests it was moved to the underground levels later on. 
The group of cables seem to be connected to elevator like structures, called scaffolds by Alyx, which are probably tasked with automated construction or mass transport of materials.
Sets of small cables also expand connection to the city from the structures like conventional power lines, suggesting the structures function in a similar way to substations. These cables were connected to rooftops by construction striders.
DestructionIn 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 and some were left to prevent the Citizens from escaping the blast. As a result, the Citadel is barely occupied compared to its earlier state.
There are however, several soldiers left tasked with a goal and others simply trying to escape, 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, debris are constantly being launched from the top. The power lines connecting the Citadel to City 17 are no longer present, and the explosion of the Dark Fusion reactor has blown off the top of the Citadel.
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 Universal Union, causing an immense Dark Energy Flare that destroyed the entire Citadel along with the surrounding City 17. Dr. Kleiner notes in one of his messages to the Resistance that the destruction of the City 17 Citadel has also caused the other Citadels across the planet to shut down.
In Half-Life 2: Episode Two, the Citadel still dominates the now destroyed City 17 even in its ruin. A Superportal looms over where the Citadel once stood, drawing power to open from the remnants of the dark fusion core through a swirling blue tendril of energy. At the end of the game, the Citadel's collapsed remains are all that is left of the once tall and overpowering structure.
Behind the scenes
The following is a Work in Progress.
You are welcome to assist in its construction by editing it as well. However, do not make major changes before consulting the community, since the related editors probably set up guidelines for the page construction.
This version is featured in the map "
c17_04_21.vmf", last edited December 2002. It is entirely brown and apparently not detailed, while a closer look shows it is detailed, featuring a circle, windows, and other precise shapes.
Its inside maps have concrete and rusty metal walls, sometimes with floor numbers on the walls, and were all last edited in 2001. One of these inside maps, "
proto_core002.vmf", shows the Citadel Core was originally to appear in Half-Life 2. Another map, "
proto_C17citadel_skybox.vmf", features a detailed spire, which appears grey-colored, not brown, and seems to be based on concept art for the previous version's lower entrance and predate the dark fusion reactor area featured at the end of Half-Life 2.
This version is also featured in three Combine propaganda posters made by Randy Lundeen (who mapped the tiles version of the citadel) and based on artwork by Dhabih Eng. In its skybox can be found the "inner wall" mentioned by Alyx in the playable Half-Life 2 Beta sound files, and where Vance Headquarters are located, as well as the radio tower located near the Manhack Arcade and Kleiner's Lab.
- Designed by Viktor Antonov, this is the last known brush version. It appears as a long cylinder divided in several parts and with other shapes stuck around it, where rooms and corridors can be found. Covered by grey and brown in the outside, it is topped by some kind of huge antenna. It is featured in the maps "
proto_citadel_002.vmf" (as a human-size version), "
proto_citadel_skybox.vmf", among others, most of them last edited in 2001.
- It is also the first version featuring inside maps. Some shapes are similar to the retail version, except all floors, walls and ceilings are entirely covered in tiles. Metallic pipes with organic forms can also be found all around, as well as a prototype prison area with Citizens and Vortigaunts in the cells, having concept art for the Depot based on them. It should be noted that most of these maps are demos and prototypes and that they are very incomplete.
- The shaft seen in the Citadel in Episode One where the Advisor leaves was originally to be seen in Half-Life 2 and is present as a prototype map ("
proto_citadel_advisor.vmf") in that Citadel version.
- Most of the WC mappack inside maps are located in that version and were made by Randy Lundeen, according to the folder name, "randy". He also designed the cut propaganda posters featuring another Citadel version.
E3 Terminal version
The version, the first model and designed by Eric Kirchmer, is a round and blueish version. It is featured in the latest version of the map "
e3_terminal.vmf". It is also featured in "
e3_strider.vmf" and "
canals_01_11.vmf". In an unmodified Half-Life 2 Beta, the model appears greenish, however this was fixed by the community.
This version looks like a giant blue-grey clamp, with Depot-like buildings at its feet. It was likely designed by Viktor Antonov. It can be found in the map "
proto_docks.vmf", probably created when Freeman was still to reach City 17 with the Borealis, "
d1_sewer51j.vmf" and "
proto_combine_underground5.vmf". The map "
d1_under0102a.vmf" contains a version halfway between this one and the next.
Pre retail version
The penultimate version, designed by Jeff Ballinger and Viktor Antonov, was first created with the same Depot-like buildings at its feet like the previous version, with the original cylinder Citadel brush replaced by a triangular prism Citadel brush (and the texture unchanged), as seen in the map "
proto_innerwall" and in an early "
e3_terminal" screenshot. The map "
proto_combine_underground5.vmf" contains the same brush as in "
d1_under0102a.vmf, but this time much bigger, and with a proper Depot model at its feet, used for the Depot seen along the Coast, and Combine mobile walls (this is a much more recent depiction). Then a model (the second known one in total) was created; this is the one featured in the playable Half-Life 2 Beta; it is almost identical to the next and latest version.
The next and last version, and the third model, is featured in Half-Life 2, and covered below. Almost identical to the previous version, that triangular prism is bluer and more detailed, and not a single Depot is found at its feet anymore.
- 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.
- The final showdown dialogue between Breen, Mossman, Eli, and Alyx was originally to be much different and much longer.
Half-Life 2: Episode One
Half-Life 2: Episode Two
List of appearances
- The Half-Life 2 Beta (Non-canonical appearance)
- Half-Life 2 (First appearance)
- Half-Life 2: Raising the Bar
- Half-Life 2: Deathmatch (Non-canonical appearance)
- Half-Life 2: Episode One
- Source Particle Benchmark (Game files only) (Non-canonical appearance)
- Half-Life 2: Episode Two
- The Final Hours of Portal 2
- Half-Life: Alyx
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 Half-Life 2: Raising the Bar
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 WC mappack
- ↑ https://www.reddit.com/r/HalfLife/comments/esen9b/were_developers_from_the_halflife_alyx_team_ask/ff9q9kk/
- ↑ Playable Half-Life 2 Beta files
- ↑ "The Final Hours of Half-Life 2", Page 6: The Hemoglobin, on GameSpot
- ↑ 7.0 7.1 Playable Half-Life 2 Beta
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