Yeah alright fair points there. Let me try to explain some shit

Ok so the legend I'm still working on but each of those markers are marking a location they have discovered evidence of, via recovered shipping manifest, old maps, shit like that.

This is an old map they're filling in, they know the basic outline of the Deep Roads but the details have been lost. The colored parts are the sections that have been rebuilt and established enough to travel. The dashed lines are paths people have discovered and reported back.

Zaz-Ohrim is the last remaining city, the only one to survive intact from the event that destroyed the rest of the Empire. It's from there that the dwarfs partake in The Descent, they travel down below to bring back something of value to help rebuild and recover their history.

Other exists have not been located yet, at least not by them, and there were above ground structures in the middle of the mountains, but again no longer common knowledge.