Undercity Dungeons and Turning City Maps into Dungeons

I got to thinking about dungeons under cities last night.  I'm not sure why exactly; I think it might've been that I've been thinking about variable corridor width since the gauntlets post, and it linked up with variable road width, the difference between broad streets and narrow alleys.

Many ancient cities were built on the ruins of older settlements, and if you repeated this process multiple times then it makes sense for an undercity dungeon to have multiple levels, with each deeper layer being the buried ruins of an older settlement.

Obviously placement beneath a population center has some nice properties for a campaign tentpole megadungeon besides just explaining its existence; low-level parties don't need to brave the wilderness to get there, the loop to resupply and return to the dungeon is tighter, and you have a ready source of rival adventuring parties.  But I think taking each layer / level as a layer of past settlement has further advantages which are less often talked about.

If you take a neighborhood as your basis for a dungeon level, rather than a single building, it might get a lot easier to manage mentally.  You can give your corridors street names and your rooms addresses.  Assigning functions to rooms should be easier here than in my traditional Dwarf Fortress-style dungeons, because you can reuse functions - each settlement was an independent layer, so each layer should have somewhere that was a granary or a forge, for example, versus a highly-vertical Dwarf Fortress-style dungeon where you have one granary or forge area for the whole dungeon.  So you need fewer ideas, and you can get mileage out of your everyday experience of the simple necessities of life as a civilian, versus trying to think of what dwarves would need in their fortresses or what evil wizards need in their towers.

I think the main difficulty might be dealing with density.  If you look at maps of Pompeii or Herculaneum, they're pretty dense, with many houses sharing walls.  This isn't how most people live in the modern West, but it would make for great dungeoneering.

Tell me this doesn't look like a dungeon map

Another interesting difference from typical dungeon maps is that when you have long, straight roads-turned-corridors, it becomes relatively easy to get close to any single point in the dungeon, versus having a mess of twisty corridors where all routes are circuitous.  I'm not sure that's a desirable property, but it might be worth experimenting with.  Certainly there were pre-industrial settlements with road patterns that would make mapping hell for the players (more hub-and-spoke than grid).  Playing with road patterns would be an interesting way to differentiate levels.

This does get away from the fantastical funhouse megadungeon and back towards the realistic ruins end of the dungeon spectrum though.  Still, it might be fun to throw a level with city-esque topology (of long, wide, straight corridors with branching narrow alleys and dense blocks of "housing") into a funhouse megadungeon.

So does anyone know of good sources of more line maps of archaeological cities like that one?