I usually run city based adventures, I find them more fun, while having far more options. There's a lot of ways of doing it, a few of the things I do listed below.
City High Level Description: Why is there a city here? Is it on a major land or water trade route (true for most larger cities)? Near valuable resources? How old is the city, how wealthy? How big? Remember that most medieval cities were not that large by today's standards and that it took a lot of land to support a city.
Also think about the racial mix and ethnicities represented. Major port cities are potentially going to have people from around the world. Think a bit about how they might interact with each other.
Maps: I rarely map things out in great detail. I start with a general idea of a city and break it up into districts. Then I write details about that district, wealth level, general types of commerce or governance and so on. There are several online tools for this, one example is medieval-
fantasy-city-generator.
Districts: This may just be a sentence or two such as "Lower Ward is the docks district, busy during the day with ships coming in and out. Also has several warehouses for offloading and storing goods. It has a red light district that is booming during trade season with entertainment aplenty for sailors."
Power Organizations and Important Individuals: I have a general idea of who's who in a city and affiliations/enemies. Again this is at a very high level at first. I may note that King Archibald is old and growing senile and his wife Clarissa is really running things or it may be as simple as the city is run by a council of the guilds and I don't even associate any names. If it comes up later I'll use one of my random lists (see below).
The important thing is to not get carried away. I generally have a half dozen groups that I make note of, what their goals are and
maybe some important individuals associated to those groups. This helps me set up conflicts and things for the PCs to do
Look and feel: this goes along with the city high level description. Take some time to think about architecture, what buildings would look like based on climate and resources. A place with a lot of wooded land in the far north is going to look a lot different from somewhere with a Mediterranean climate that has access to few trees but plenty of high quality building stone.
Random lists: Since I rarely write down exact details, I rely on random lists for individual names, business names, names for thieve's guilds (there's likely more than 1) and so on.
Once the campaign starts, I work on a starting area and put in a few more details as I plan and go along. It's a lot like campaign world building, you have a high level picture but really focus on a small piece first. In a city campaign I really only think about the district or even the neighborhood the PCs are in and slowly expand out from there.
