Never mind, I found a brush of it.
That isn’t the beta citadel, this is. http://half-life.wikia.com/wiki/File:Proto_citadel_skybox_fixed0015.JPG There’s a model of it somewhere in the HL2 Beta files.
I think that’s brushwork, actually. Could be wrong though.
There are a lot of different designs for the Citadel, while only one of the scrapped ones has a actual model.