I think more than any location in RPGs I'm drawn to the city. I don't know whether that's because I'm a city dweller, but there's something about a heaving metropolis that fires up my synapses and sends my imagination into a frenzy. It's the dark, twisted streets, the bustling markets, the palatial homes of the aristocracy peering down their noses at the slums of the fevered peasantry. It's the alleys and the dive bars, the parks and the breweries. A world away from the lush green landscapes adventurers often traverse.
To me a city in an RPG has to have a character of its very own. It lives, breathes, sees and sometimes even dreams. Look to examples like Discworld's Ankh Morpork, Batman's Gotham or Fighting Fantasy's Port Blacksand. Each has its own personality and way it operates. Even the brief overview of Highbridge in my own Halberd Fantasy Roleplaying goes a long way in showing what kind of a city it is.
I hate a clinical city. One with bright buildings where the sun always shines and the populace is bland. Don't get me wrong, there's a place for this kind of city, but I take mine without milk and sugar. I love dark places where death is a possibility any night of the week, where the cobblestones have been worn from horse and cart commerce and where the upstanding gentleman is your enemy and a cut throat street urchin an ally.
So what does a good, the thriving game city need to captivate its players?
-Notable NPCs: whether it's blind Baron Von Wildenwrath or Doris Ealy, the clairvoyant maid, your city should have at least 10 NPCs of note with their own back stories and quirks.
-Conflict: Your city should have both inner and outer conflict. Create 5 main factions within the city walls, each with their own motive. They could be good or bad. Secret societies, gangs and even unions can be the source of conflict in a city. Then think about those outside the city who could be a threat, such as raiders, armies, dragons etc.
- Politics: this can also cross over with conflict, but you should figure out how your city is run. Rarely should the ruler be benevolent as that's frankly less interesting. Greedy mayors, tortured advisors, well-meaning but weak statesmen. Is it a democracy? A pluticracy? Maybe a dictatorship? Is there the smell of revolution in the air?
These are just some examples of what a good city needs. Let me know in the comments if there is anything else you think belongs on the list.