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 dialogue from Kleiner in Episode 1 says that there are multiple citadels on Earth, though an exact number is never stated.
Unlike City 17, the Citadel is a purely Combine creation. Tall, 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 limited to essentially polished metals, metal grates, glass, and marble. 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. 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 is very monochromatic and cold, with tinges of blue and teal throughout the level. As with all Combine lighting setups, strong 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.
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. Because of the development of stalkers' code, they should not be in a position where the player can attack them, unless their code is fixed (Of course, Stalkers]] will be fixed by Half-Life 2: Episode One, since this is one of Episode One's features). 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.