Citadel
The conclusion of 
Half-Life 2 takes place in the Citadel, a very tall, monolithic tower embedded in the middle of City 17; it is basically the quintessential symbol of Combine might. From the very beginning, the Citadel forms a very strong relationship with the player, as it looms over the cityscape and fades into the clouds. When the player actually travels into the Citadel alone in the final levels, there is a great sense of closure, leading up all the way to the portal array at the summit. Some monologue from Kleiner in 
Half-Life 2: Episode One mentions that there are multiple citadels on Earth, though the exact number is never stated.
Architecture
Unlike City 17 and most of the previously seen Combine establishments, the Citadel is a purely Combine creation. Its features are almost always tall rather than wide, evoking a strong sense of verticality in both directions, and asymmetrical angular forms with an unsettling approach to pattern are very common, meant to make the player feel insignificant against the Combine war machine. Materials are essentially limited to typical Combine metal walls, polished metal tiles, metal grates and reinforced glass. Examples of the Combine technology found in the Citadel include Combine computer terminals, sprawling yellow pipes, force fields, power generators with dark energy orbs, and stasis fields of unknown purpose. Razor Trains can be seen arriving to loading platforms located in the lower levels of the Citadel through underground tunnels. As the player travels upwards, the setting becomes noticeably more "elegant", until they reach Dr. Breen's office with its luxurious red carpets, wooden furniture and marble floors.
Environment
To emphasize the alien nature of the Citadel, the overall color palette follows a strict monochromatic and cold pattern, with tinges of black, white, blue and teal throughout most of the levels. A reasonably dense bluish fog should always be used to visually enhance angular silhouettes and deep dropoffs ubiquitously found in the Citadel. Additionally, in the concluding stages of Half-Life 2, the cold interior is contrasted sharply by the orange sunset outside. As with all Combine lighting setups, bright teal spotlights are very common. Since the structure is entirely self-contained, windows should have a very strong white glow, creating a feeling of a strong contrast between the outside brightness and the internal darkness, but different approaches are also possible, as with non-obscured windows overlooking the orange sunset in the latter stages of Half-Life 2.
It should be made clear that the player is an intruder and is not supposed to be there. In fact, Valve originally had citizens firing rockets and assaulting the Citadel, but felt that the player travelling inside alone would elicit more emotion and evoke heroic tones.
In Half-Life 2: Episode One, following the Dark Fusion Reactor's destruction affecting the whole Citadel, the interior color palette is dramatically inverted, with the original cold pattern being strongly contrasted by the orange and yellow lighting coming from burning structures along with dense orange fog. The Citadel appears irreversibly damaged and in the process of inevitable, chaotic breakdown: most of the Combine technology is broken or malfunctioning, and burning metal chunks constantly fall down from the upper levels. The urgent atmosphere is further enhanced by the sounds of metal groans, distant explosions, blaring sirens and nonsensical, repetitive Overwatch Voice warnings provided by soundscapes. The Citadel Core chamber is also seen for the first time, predominantly utilizing unique white-colored Combine metals. Another newly-featured area, the Advisor pod room, provides a niche aesthetic with its all-red, thin light spots.
NPCs
Seeing as the Citadel in City 17 is the global Combine headquarters, only Combine NPCs should be present. Lines of Striders, Gunships, Dropships and Soldiers (both standard and elite variants) can be seen, as well as Stalkers. Originally, because of Stalkers' model missing ragdoll state, killing them resulted in a crash (and the original HL2 model still simulates incorrectly today), though this has been officially remedied since Half-Life 2: Episode One introduced an updated model. Scanners can be seen circling around in open areas, and Manhacks are deployed in case of intruders. Dr. Breen and Advisors can make appearances in scripted scenes. Other Combine synths such as Crab Synth and Mortar Synth can be glimpsed, but player interaction should be avoided due to these NPCs being incomplete and reserved for scenic purposes. As seen in Half-Life 2: Episode One, Rollermines are sometimes transported and contained in glass tubes, but can be let out of them with a press of a button. Note that the giant pistons at the bottom ensure that antlions will stay away.
See also
- Citadel soundscapes
- Half-Life 2 Sky List - See maps
d3_breen_01and d3_citadel_01 throughd3_citadel_05.
Reference
Citadel
Citadel - Foot
Citadel - Dark Fusion Reactor - Bottom
Citadel - Dark Fusion Reactor - Top
Citadel - Inside - Energy Balls
Citadel - Inside - Energy Balls
Citadel - Inside - Energy Core Room
Citadel - Inside - Pods
Citadel - Inside - Tubes
Citadel - Inside - Tubes
Citadel - Inside - View
Citadel - Breen - Office
Citadel - Inside - Walls
Citadel - Inside - Walls
