There are other citadels on Earth. Also, in looking for the source I discovered that they are all currently not functioning properly due to the destruction of the Citadel of City 17. They are, however, still there. They are mentioned in the Kleiner-cast in episode 1: "The destabilization of the City 17 reactor has had repercussions that were not entirely unexpected, although we hardly dared speak this hope ahead of time. The destructive pulse forced a damper on the entire network of linked Citadel reactors. Thus, for the time being, I believe that all Combine portals have failed completely, as well as all communication systems based on that technology."