[Anaraxes]

A topopolis hasn't been mentioned in the thread yet. You might have multiple distinct ones braided together in the same space, but not connected, perhaps for different environments or just for "redundancy" as an excuse to have separated cultures or peoples. Travel outside, between tubes, could be good for shortcuts compared to the usual inside paths for long distances, and so narratively function something like teleportation or ley-line / Nevernever / the "Ways", etc walking in fantasy stories.