We passed doors to several other levels on the way up, but finally all the drones were at the top of the stairs. As Gandalf described it, the door opened into the foyer of the transit station on the inside surface of the megastructure. In better times, this was where the inhabitants would have come to travel to the outer shell and then outside the topopolis. There was some indication in the scans that this was also a transit stop for some kind of internal transit system. Unfortunately there were only so many milliseconds in a day, and that investigation had been back-burnered.