So, before we move any further, let's define what a city campaign is exactly. The city campaign is one in which the majority of the players' adventures take place in, or around, a particular city, or cities. Before determining if you're going to run a city campaign, meet with your players and discuss the idea with them. The city campaign isn't for everyone, and the last thing you want is a player upset because he feels "restricted" to adventuring within a city's walls. If the players know beforehand, they can create their characters in a fashion more suitable for adventuring in an urban environment.
Before you set down to design your city, you must first determine what kind of city it will be. Will it be a large metropolis (pop. of 100,000 or more), a large city (pop. of around 50,000-90,000), or a smaller city (pop. of around 10,000-40,000)? What kind of races inhabit the city? Where will it be located? Perhaps at the foothill of a mountain or on the shore of a large river? Maybe it will be a port city or one that rests on the border of a kingdom? City placement is important, especially in building your world. Cities tend to develop where it makes the most sense; either along a large trade route, on the coast, etc. Think about where the city is located, and what impact it has on the world overall outside the city gates.
How is your city's government structured? I personally recommend the independent city-state, but there are a variety of options here. Perhaps the city is part of a larger kingdom ruled by a malevolent tyrant. Maybe it's run by a cabal of mages or priests, or a merchant's guild. This is an important aspect of designing your city because it offers the possibility of creating great adventure hooks for the PCs down the road. Think about the ruling structure and how that will relate to the characters involved in the campaign.
All cities thrive on trade. Trade is the key to civilization in the fantasy setting. What types of goods and/or services does your city provide? Is your city located near the base of a mountain? Perhaps your city's primary trade is equipment crafted from ore taken from the mines outside the city? Perhaps your city is an even greater hub for the neighboring cities around it. Maybe merchants flock to the city's gates to sell wares from all across the lands in your vast city markets. Maybe your city is home to the only known sorcery academy in the region and is the center of your economy. Once you've sorted through some of these things, the picture of your city should start coming into place. You'll be surprised at how the creative juices start flowing when you jot down just a few ideas on paper.
Most fantasy cities are divided up into different sections. These wards and burroughs serve as a way to divide the city's lower, middle, and upper class members. If your city is along a coast, you'll want to establish a dock ward and think of the various ways to describe it. Maybe the dock ward is the hub of your city's industry and thus patrolled regularly by the city watch, or maybe it's a harbor for fugitives, pirates, and other villains. Merchant or business districts are also important, as this is where most of the city's industry will take place. These districts should contain various specialty shops which can't be found anywhere else. This is a city, after all. Be creative in how you divide up your city and how you determine the comings and goings within.
At this point, you'll want to consider some basic laws particular to your city. For example, will adventurers be required to peace string their weapons upon entering? Will they be required to register and pay taxes within the city as "adventurers?" What is the punishment for a bar room brawl? How is your city policed? Are the constables or city watch corrupt?
These are some basic ideas to get you started on fleshing out a quality city campaign. In the next few posts in this series, I'll detail how I go about filling these cities with interesting places and people, designing maps of the city, and creating some adventure hooks that your players can sink their teeth into.