Funnily enough, I was an anthroplogy major before switching paths, and I've done a fair share of archaeology, too. In fact, I was thinking of adapting some stuff from greek archaeological sites (particularly some of the Cretain ruins) for my ruined city.
My main concern with this post was, in fact, how to map the silly thing. I have the location pretty firmly established in my mind.
But my best advice is do not become entrapped by the idea of thinking of a city as just a 2-Dimensional grid or space. It's at least a 4-Dimensional "Super-structure." So it's useful to map it that way.
Good luck with your design and project.
Very good advice, and consider it heeded.
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I guess I should give a bit more detail on what I'm doing, and how it will be implemented.
The city is called "The Mazeworks", and was once a series of canyons and grottoes eaten away through rainfall - an arid region of limestone. The local Minotaur tribes used it as a congregating site for their quasi-religious astrological gatherings (minotaurs, in my world, being very anti-religious). The Tiefling empire (back before they were technically "tieflings", for all intents being roman-like humans) established a base here, to use it as a northern fort city and trading post, and named it "Kael Tessera".
While there, the imperials had to deal with frequent minotaur uprisings. Much of their "city" was built over the mazeworks, while the minotaurs lived in the lower levels (which became a lower-class city). The imperials would descend "into the maze" during the day, but would quickly come back up when night fell, as minotaurs used the dark to go hunting. So, the lower levels are a mixture of natural caves, the remnants of shanties, and impromptu greco architecture, while the upper levels are built almost like, say, Venice. Many nice buildings, connected by bridges, with frequent guardhouses, warehouses, and markets.
With the demon scourge, the place was flooded with devils and demons (they are functionally the same thing, in my setting). The minotaurs fled to the hills, and fragmented into feuding tribes. Much of the city is plagued by undead (old citizens), devils, and the ghosts of minotaurs.
The goal of the PCs will be to get to a specific building, while also dealing with a rival (an expedition from one of the Tiefling city-states to the south). I plan on making this expedition take several days, if not weeks, in a sort of skill challenge.
I like the idea of a basic city map, though I guess I'll probably make the "maze" portion a skill challenge. I was thinking using a wide-scale map, and dropping numerous individual buildings that would be so-called five-room dungeons (each giving a clue as to the location of the main building, rewarding exploration).
There would also be random encounters (devils, minotaur ghosts, feral minotaur exiles, undead, tiefling legionnaire squads, various animals and solo monsters, as well as natural hazards and role-playing opportunities).
The idea of "Street grids" is a good idea for when an encounter happens, as I can just place them together on the fly. If I do it right, and keep things hidden, it will look as if I've actually mapped the entire city, which could be really cool.
And Celebrim - I love that "random find" table, though I'm unsure whether I'll factor it into my random encounters table, or make it an individual thing. I only have a few random find ideas, right now, though.
Keep the ideas coming!