On the topic of verisimilitude, here's an excellent article about demographic calculations: http://www.io.com/~sjohn/demog.htm Especially note the calculators linked from the bottom of the page.

Knowing how many of a given professional should be working in a city of a particular size can help in allocating space for various neighborhoods. In the case of a planned city, knowing just how many smiths there should be in a city of 12,000 will help you to know how long the Smith's Way should be. In a more organic city, you can spread out some basic professional, such as the bakers, and build your neighborhoods around access to that particular crafter.

When designing a city, I typically start with the history: why did people settle here, and how did they build at first? What caused a small settlement to become a large city? How did those forces shape the way the city grew?

There are a number of factors that might go into such questioning: its military significance, the economy of the area, the personalities of its rulers, its proximity to other cultures/rivals, local terrain, building resources, weather, food supply. Any or all of those things might need to be considered.