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.
The Nexus architecture is like a museum or a fancy administration building (city hall?). There's a lot of Combine technology in here, like heavy steel doors, turrets (ceiling and floor varieties), and energy containment modules (those things with the Combine balls floating around in them). It's moderately fortified on the outside, with the usual energy fences (with conveniently located plugs) and regiments of soldiers.
The Nexus is in the middle of a small square, surrounded by some residential or industrial buildings. Once out the door it's a game of cat and mouse as you and your buddies try to avoid being suppressed by the suppression device. There's a variety of cover to use, like an overturned dumpster (don't ask how it got out into the middle of the street), a wrecked car, and the awnings of a building (apparently buildings in City 17 are indestructible).
The nexus has a large courtyard in front of it, with some grass, trees, and park benches. It's surrounded by a tall wrought iron fence, with an energy fence where the gate would be. Fortunately, an inept soldier seems to have left a door to an adjacent building unlocked, allowing you access behind the gates. Then it's back to running from the suppression device again, across the courtyard to another building where you can replace the rebels that got suppressed on the way there.
After that and your adventure inside the nexus, and after you emerge onto the rooftop, a full on street war erupts in the streets below, with Striders being air-dropped, and the sound of gunfire erupting all around you.
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 Episode One, it should not be possible for players to attack Stalkers due to their incomplete implementation. There is also a suppressor on the roof, which will disintegrate anything that it hits, via laser.
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). Striders may also be carried by dropships, if neccessary.
To do: Expand?