You will have to be more specific regarding what time period you are looking at and what country.

In general, social mobility has never been so great in the past but that isn't to say that it wasn't there. The chances where better if you were of the city rather than the poorer outlying areas but again, this is specific to the period and location.

Ann Wroe's A Fool and his Money is a good snap shot of what a city in Medieval France could have been like.

I have to remove this question for falling afoul of our throughout history rule. It is true that you specify the "middle ages". However, that is a quite broad range of time that can encompass anything from the fall of the Western Roman Empire in 476 until the 1492. Additionally, you are unspecific in terms of the society you are talking about.

You are free to re-formulate the question, but please be more specific to century and area.