I built-up a sliding back wall for the highrise.
It's fairly easy to make. There're 3 layers: there's an inner piece of black mat board, a middle layer of 0.100" styrene strips to act as a spacer layer, and an outer layer of black foam board. There's a track on the back ends of the side walls - it's those white strips in the photo - made from 0.080" square styrene that the back wall slides into. In the photo the wall is partially slid up to reveal the lobby and lower floors, but the entire wall can be slid out to expose all the floors.
After pondering things a bit, I need to build up the lobby next so I can get the bottom of the building firmed up and detailed.
The back wall isn't going to be left as raw foam board, but will be finished simply with styrene brick sheets and a few detail items. The details won't be prototypically accurate, just suggestive so that a casual glance isn't too jarring.