Charles Ryan wrote a very approachable piece on the population and structure of the Medieval Kingdom, for some assumed value of ‘medieval kingdom’ that seems to be common among gamers. He describes population density, settlement density and distribution, and feudal structure.
Including mentioning that “feudal structure” is a bit of a mess and nowhere near as simple as many people seem to think. I remember talking about this with Joseph Browning at GenCon 2003 (Magical Medieval Society: Western Europe had just come out) and it was somewhat enlightening.
Did you know that it would be entirely possible for the King of England to be required to provide troops to fight himself?
See, if Henry has land (probably managed by a seneschal) in France while he rules in England, and France declares war on England, then when Louis sends out the call for troops it will bubble down until Henry’s seneschal gathers the men required by the feudal agreement and sends them to Louis, who puts them in a boat to England to try to defeat Henry.
Thus le baron Henri could be required to provide troops to fight King Henry of England. Who needs subtle machinations and cunning plans to confuse things?
There is more information on the subject available elsewhere for the morbidly curious, but this post is a straightforward and approachable treatment that will probably be sufficient for verisimilitude for most people.