OOC: A quick response to kipper
This particular megadungeon 'sprawls' horizontally rather than vertically. That is to say, "dungeon level" is measured by travel distance north/south/east/west from the exit (as opposed to depth below the surface).
There are vertically deeper areas to explore (such as the Grand Chasm) but it's best to think of that as a wilderness zone, outside the "dungeon level 1-10" nomenclature. When you are ready to go down there and battle purple worms and such, I have sketched out a few adventure hooks.
The transitional areas are well-marked, and you don't have to worry about your PCs ever wandering out-of-bounds by accident. (Unless of course you ever run out of light and start stumbling around blindly; then all bets are off!)
As I was planning out this campaign, I was intrigued by the idea that maybe dungeons have alignments like people. There's this concept of the "mythic underworld," that left to its own devices, the dungeon's natural tendency is to revert to a state of primal Chaos. And then I like to think that the act of Adventuring, of shining your lantern into the darkness, drawing a map, and slaying the monsters, brings Law to Chaos. So therefore logically, Level 1 of a frequently-explored dungeon, would have an alignment of "Chaotic (Lawful Good tendencies)" that could manifest as friendly messages from past Adventurers, written in chalk on the walls, or helpful equipment lying discarded on the ground. As the party explores higher-numbered dungeon levels (not necessarily deeper into the earth, but farther from the entrance) these Lawful echos of past Heroes will fade, and the dungeon will get less "friendly" and more Chaotic. For example there will be fewer helpful hints and 'Easter eggs' to guide the party.
And then the Underdark is something else entirely. It has more of a True Neutral energy, that the creatures down there are just trying to survive, and they see you as protein.