Here I did some quick colour roughs of a type of tram station, that utilises high speed transports for shipping the prisoners to the different sectors of the prison.
Although I liked the concept It was pointed out it focused to much on the environment and did not show much of anything regarding the prisoners.

My second idea borrowed from the first by having the aliens being roughly shunted through the main entrance to the prison.
The main changes consisted of making the doorway a much more interesting shape and allowing for a grate number of vastly different shaped aliens to pass through its foreboding archways.
I completely dropped the idea of the trams at least for this image, to better focus the attention on the aliens and guards in the scene.
In the background you can see I brought back an idea I drew up previously of the shuttle that had dropped off the prisoners which is now blasting off back to the space stations in orbit.