- We have the table I did that allows us to quickly define hamlets, villages, towns and cities with a handful of labels and die rolls, plus the rules that let us define individual trades as needed, instead of in advance.
- We have the quarter system as a generic town and city layout; it can be modified slightly to provide other layouts (use parallel strips instead of quadrants around a hub, drop the merchant/noble quarter for smaller settlements.) It even gives us up to 21 neighborhoods.
- We can roll for the generic town type, derived from the NPC types, to determine the general character of a town or city's inhabitants. We can even roll this for each neighborhood to provide more detail (don't go in the temple district, those priests are schemers!)
- We have Zak's technique in Vornheim to define major thoroughfares in a neighborhood as needed, instead of mapping them out in advance. You can use other words, such as keywords describing each neighborhood or Roman Numeral dice, instead of spelling out the names of the numerals every time.
- We have another Zak technique to define the layout of an unmapped building: roll a traditional d6 with pips instead of numerals and use the pip arrangement as the room arrangement. Maybe roll 1, 2 or 3 dice on a sheet of paper to define more elaborate layouts on the fly.
- We can modify Zak's room layout technique to define building layouts in a given block. Roll one or more d6s; each pip on the die represents one building, and the arrangement of pips is the arrangement of buildings. A 5, for example, indicates four buildings on a block surrounding a courtyard that contains a fifth building.
What we can do with all this is define an entire city in a matter of minutes, using a few paragraphs, and defer all details until later. You don't need to define *any* towns or cities in advance; all you need to determine are the name, main economic activity, and distinctive feature(s) of the city, and everything else is random or improvised. Even the name, economic activity, and features can be randomized, with things like random medieval occupation lists or my 20-Sided Quickie tables or Risus Monkey's DungeonWord tables.
As the adventurers explore the city, they will discover more details about it, as much as they desire. When the GM has an interesting idea for an in-city location, it's an easy matter to map it and drop it into an appropriate neighborhood, wherever it will fit without disturbing what the players already know.