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 Episode One mentions that there are multiple citadels on Earth, though the exact number is never stated.
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. The bottom portions contain lots of metal substructures, possibly to support the giant pistons located at the bottom of the Citadel. As the player travels upwards, the setting becomes noticeably more "elegant", until Dr. Breen's office with its luxurious red carpets, wooden furniture and marble floors. The height of the Citadel from surface level is about 2 km, the underground structures adding about an additional 0.6 km to the tower's overall height.
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 Episode One, the interior color palette is dramatically inverted following the Dark Fusion Reactor's destruction, resulting in the usual cold pattern being almost completely replaced with orange and yellow lighting coming from burning structures and complemented by dense orange fog. The Citadel Core chamber is 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.
Seeing as the Citadel in City 17 is the global Combine headquarters, only Combine NPCs should be present. Lines of Striders, Gunships, 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. Dr. Breen and Advisors can make appearances in cutscenes. 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. Note that the giant pistons at the bottom ensure that antlions will stay away.
- Citadel soundscapes
- Source Materials Sky List - See maps d3_breen_01 and d3_citadel_01 through d3_citadel_05.