Charles Ryan wrote an article back in November 2011 about the Medieval Kingdom. I missed it at the time, but it is a very approachable summary of population and settlement demographics in a nominally medieval (i.e. “what most people seem to think of as medieval”) setting.
This includes a brief description of feudalism, including mention that it is really nowhere near as simple as most people seem to think.
I remember talking with Joseph Browning at GenCon 2003