The denouement 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. By the end of the game when the player actually travels into the Citadel alone, there is a great sense of closure, leading up all the way to the portal array at the summit.
Unlike City 17, the Citadel is a purely Combine creation. Tall angular forms are very common and make the player feel like an insignificant speck in an unstoppable war machine. Materials are limited to essentially polished metals, marble, and blue-tinted concrete. The bottom portions contain lots of concrete / metal substructures, possibly to support the giant pistons located at the bottom of the Citadel. As the player travels upwards, the setting becomes noticably more "elegant" until Dr. Breen's office with fancy red carpets.
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. (add more about lighting, if there's anything more to add)
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 human slaves. Note that the giant pistons at the bottom ensure that antlions will stay away.
(I'll add some later. I promise!)