Combine headquarters near the Citadel. Equipped with a suppression device that... well, suppresses. There is likely one in every major city that has a citadel. Any cities that were small enough to avoid getting their own citadel, assuming they weren't all cleared out by the combine, would also have a similar Nexus building in place of a citadel.
To do: Architecture
To do: Environment
As a Combine headquarters, the Nexus should be populated with combine police and soldiers. Striders patrol the streets close to the Nexus. Floor-mounted turrets triggered by lasers are positioned to guard key points (such as forcefield generators or the pathways to them).
Stalkers might be kept in the Nexus to perform day-to-day tasks and repairs, etc. None are seen in Half-Life 2, although that doesn't mean they aren't there. Remember that, until Aftermath, it should not be possible for players to attack Stalkers due to their incomplete implementation.
Rebels are located throughout City 17 and around the Nexus is no exception. In your levels you might want to allow rebels to help you or simply stage a battle, if appropriate for your story.
In case of an assault, such as in Half-Life 2, it is most likely that the Citadel will dispatch gunships, dropships with combine soldiers and elites, and striders (via bridges between the Citadel and the rest of the city).
To do: Expand?
To do: Post good inside pictures