Well, just off the top of my head - The castle usually was built around a Keep that was in a strategic location such as on top of a hill or some defensible position and things went from there. The wealthy would want more security and pleasant surroundings so would probably be in the next best location behind a couple walls or something. They would also probably have more dealings with the government and thus would logically be closer to the king's areas (I'm sure there are all sorts of exceptions). Moving on down the line you would clearly have the poor in the crappiest of areas, probably closer to the dirty industries and least protected areas. Besides, there was a real class issue back then and the wealthy could hardly stand the poor around them so far away was best. Merchants would want access to shipping and roads so they would most likely be found near those areas.
Pretty much any planning and zoning would be done for some type of logical reason. The logic may have changed over time but there would still be some reason for the original locations used.
I think I recall something along these lines being a big thread back around the time I first joined the Guild but I'm not sure.
On a side note, here is an interesting article on mid-evil demographics <-- not exactly what you are asking but still might be good information for anyone doing this.