Underside was just a series of vents that connect different parts of the Warrens to the Others. Not truly part of the Warrens, but part of it's infrastructure. Being infrastructure, probably has a definite top/bottom.
The DL's are probably located at random points of the vents; only connected via vents/portals(possibly even teleporting to increase randomness) Meaning it would be impossible to know which one is at the top/bottom. SubDL's are most likely older parts of the dungeon, which have been repurposed into infrastructure.
If that's the case, why are there people in the Underside? And, for that matter, where is everything?
1:Proxxy has a heart and decides to throw intruders into the vent system rather then purging them from the system.
2:Everything is located among the vent systems. How else would the randomly teleporting DungeonLevels get any oxygen? Or simply floating around in empty Voidstuff.
Well, this brings up another few questions:
1. Why do the life-support systems of the Warrens include enough room for people to live in?
2. Where did the materials that the Undersiders turn into coins, items, buildings, etc come from?
3. Why are there people in the Warrens? Other than the minions of the Siblings, I mean.