We often read that King so-and-so, or Lord so-and-so "raised an army". What were the details of that process in medieval Europe (12th century England to pick a specific example)?
My understanding is that there was a hierarchy of vassalage (though the term "feudalism" is problematic) from the monarch through various levels of aristocracy and other landowners down to the peasants and cottars. Is that idea of a hierachial system accurate, or was it more complex than that?
Who was selected for levy, who was spared and why (family structure, needed for other work, favouritism, previous service)? To what extent were levy troops willing and to what extent were they compelled? What was the mechanism for ensuring a functioning agricultural output while supplying troops? How did the call to arms get passed around the geographical area, and how was a coherent body of troops gathered and directed?
I can't give you one on 12th century England, but I can give a rough outline for 14th century France.
It depends on the size of the force, first off. Let's say we're going to go full-blown here. The King's guaranteed forces would be his knights, as this was pretty much their purpose in life (on paper). After that he would want to hire mercenaries.
Mercenaries were always preferred over peasants due to their training. The average peasant had virtually no training whatsoever and was therefor considered practically useless. Mercenaries were paid for using the King's money generally raised from his nobles and (if necessary) taxation.
Since we're going tout-neuf in this example, i.e. English forces are arriving on French shores and generally causing huge problems, then the King could issue something called an arrière ban or general summons. I'm going to quote Barbara Tuchman's A Distant Mirror (my go to source for this) for a bit here.
Based on the principle that all subjects owed their lives to "defense of country and crown," the general summons was suposed to be used only when the call to nobles had not or would not suffice to repel the enemy. It was issued, like all public announcements, by "public cry", that is, by heralds riding forth to proclaim the order aloud in market place and village square.
From here the towns had a choice to make payment via actual bodies (this being unpopular at the time) or in money, to hire more mercenaries.
The system for designating how much one had to donate was measured by a town's prosperity and number of hearths. So if your village was doing well, in that case you need to supply or pay for one soldier for one year for every 100 hearths your town has. If you live in a not so nice village, it might be more like one soldier per every 200 or 300 hearths. As one might imagine, this is not very many soldiers at the end of the day.
As for direction, there was no clear rank of leadership as there is in today's military. Instead you had one or two men in charge below the King, and the King's councilors who would meet and discuss battle plans. Authority over smaller groups within the army itself was pretty sketchy and generally fell upon the soldier's loyalty.
I hope this answered some of your questions.
Edit: I found a little on how the English system worked, this being according to Parliament authorized war subsidies in 1345. The responsibility for supplying troops fell upon landowners as follows. A man with £5 of income (that's annual income) gave an archer. £10 gave a mounted spearman, £20 gave two of these, and over £25 supplied a man at arms - this being a squire or knight.
There's an interesting note at one point in the book Voices of Moorbath. A tiny village in Devon, England, the town is called upon as one of many to send troops off to war against France. This would be considered more Renaissance era though, but it's still interesting to read about the politics of the king calling for war.
On one occasion, the town vicar notes that the villagers were able to pay for armor and weapons for two men to war, and notes that it was a substantial drain on their finances.