Locations/The Citadel

The Citadel, seen in every city under the Combine control, is an invasion central from which the Combine used to conquer entire cities. It likely emerges through a Massive Portal placed on the city's underground.

The only Citadel visited by Gordon Freeman is the City 17 one (see Citadel on Combine OverWiki). It exploded after the events of Episode One and lead-up to Episode Two.

In P:AAA
The Citadel visited in P:AAA will be of the city which has no definite number for the whole world.