Fleshing Out The Campaign World

As a GM, you always have to keep in mind that your players won't necessarily do what you plan on them doing - especially in a city, which lacks the GM-friendly constraints of your average dungeon.  In such cases, you will have to improvise.  It can be tricky to come up with something at the spur of the moment, but having a fully-realized and fleshed-out game world really helps out with that sort of thing.  If you have established the history and culture of your city, not only will it feel very real for your players, but it will be so real for you that you will have very little trouble improvising within it.

For example: