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 Citadel Core chamber predominantly utilizes unique white-colored Combine metals. 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 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 setups are also possible, as with non-obscured windows overlooking the orange sunset in the latter stages of Half-Life 2.
In Episode One, the interior color palette is inverted dramatically 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.
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.
Seeing as the Citadel in City 17 is the global Combine headquarters, only Combine NPCs should be present. Lines of Striders, Gunships, and Soldiers 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. 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.