It's important to remember that "medieval" covers a very broad span of time, and even if you just look at Europe as an example, there is a HUGE variety of city styles across areas and time periods. For example, in the Roman period, city planning did exist. Roman engineering was extremely advanced, and at its prime the Empire was able to assemble a large enough force of soldiers and engineers to build very well constructed roads, sewer systems, aqueducts, etc. that were still in use over a thousand years later! So if a "medieval" city was originally built by an ancient empire (like Rome), it's possible that the heart of the "old town" would have very geometric street layouts, it might have some sort of underground sewer system, some type of plumbing system, etc. Particularly if the setting involved magic, or dwarves (who are usually depicted as having some mechanical engineering ability), these ancient systems may have at least been maintained at a serviceable level, even if improvements or replacements were not made. Likewise, if you have a medieval city which began in a "dark age" but then later that country/region experienced a sort of "renaissance", then older parts of the city might have been demolished, and a newer more orderly section of the city laid out and built.

Cities are almost like organic life, in that they grow and expand, or shrivel and shrink and die. Some cities are more orderly than others, depending on the general culture in which the city is founded within, or what culture it grows or shrinks within. Environmental factors have a definite impact on whether a city grows or shrinks, or whether it is abandoned altogether (making those awesome ruins adventurers love to explore!). So think about your culture, think about the environment, think about the level of technology (if it is low level and people are restricted to horses for transport, the city will be much smaller and settlements will be closer together), think about government systems and how much support there is from the regional/national government in terms of security or infrastructure building...all of those things can influence how you build your cities.

On the other hand, it's fantasy, right? Don't get so caught up in making it "realistic" that you never actually finish it! Good luck!

--wisemoon