Hello! Medievalist here! I specialize particularly in 12th C England and France. Things do change towards the end of the Middle Ages, when you start getting models of dedicated, full-time military forces.

I answered a similar question HERE discussing procedures for raising levies. I will be reusing some of this material, but I want to approach this from a different angle.

In the 12th C, military service was largely confined to the aristocratic elite by their cultural values and political institutions. At it's core, the structure of medieval aristocratic society was based in the obligation of economic and military service to an overlord. Sometimes these relationships could get quite complicated, as there were many layers of feudal obligation at work. However, this was not a one-way relationship. In exchange for taxes and military service, an overlord was meant to provide protection, stability, and consistent administration of justice. That places military service in the middle ages in a grey area where, while it is involuntary, there is some compensation given for this type of service. However, the notion of Kings or nobles passing through a town and summoning any able-bodied man to fight at a moment's notice would have been extreme in the 12th Century.

In England particularly, there was a sophisticated system of knowing how many men could realistically be fielded by each vassal based on their holdings and income. It is telling that both William I and Henry II, on coming to their thrones in 1066 and 1154, begin their reigns with essentially an audit of their new holdings. Philippe Contamine discusses the procedure of Henry II's 1154 audit:

They were asked four questions: how many enfeoffed knights had they at the death of Henry I (1135)? How many household knights had been enfeoffed since then? How many knights did their demesne properly owe? And, what were the names of the enfeoffed knights (this last information would allow the king to check those who had not taken the oath of allegiance)? Replies were contained in the famous Cartae Baronum: 283 tenants-in-chief declared there a total of 6,278 & 7/8 knights' fees, though the servitium debitum [service owed] only amounted to some 5,000 knights - 1,000 supplementary knights furnishing, one might think, a sort of safety net taking into account minors, old men, the infirm and sick. The abbey of Bury St Edmunds, for example, had 50 knights dependent on it although its servitium debitum was only 40, and the abbey of Abingdon, which owed the king 30 knights, possessed 33 feoda militum.

Here you can see the practicality of the accounting in this period. And the discrepancy between the total, counted knight's fees and the final servitium debitum indicates that there was room for leniency and extenuating circumstances. Furthermore, Stephen Morillo expresses doubts that this system was actually practically used as a means of summoning armies together as there are limited accounts of this system actually being invoked. What was more common, in his view, and gaining traction throughout the 12th C, was the practice of scutage. According to this practice, instead of supplying men and arms, you could instead make a payment to your overlord based on the evaluation of your fiefdom. This money could then be used by your overlord to field men, often mercenaries, but also more local men who saw that this type of service could be profitable.

Scutage could have multiple layers to it as well depending on the needs of the Kingdom. The King may allow low to mid-level nobles to fulfill their obligations to their overlords through scutage, but still require his tenants-in-chief to supply a certain portion (or all) of their obligations with armed men rather than a cash payment.

There were also limits as to how long service was required. The King of England could only require 40 days and 40 nights of service. This had been decreased from 60 days at the beginning of the 12th C. However, there is an implication that, if a campaign is not finished by the time the 40 days has expired, conscientious nobles ought to stick around out of a sense of duty to their King. However, there were also potential benefits to continuing service beyond one's obligations. The King might recognize such services with gifts of money, land, titles, or by granting personal or political favors. It was also seen as honorable, and would be looked upon favorably by the noble's peers.

The image of the poorly-equipped, unwashed armies of peasants wielding repurposed farm implements is one that pervades depictions of the Middle Ages on screen. But it is not really accurate. Certainly peasants were crucial to war, as they would have been brought on campaigns as blacksmiths, cooks, laborers, grooms, etc, but they were not a major force on the fighting field. While not all combatants were aristocrats, there was at least some bar to entry for warfare. Henry II's Assize of Arms stated: "Let every holder of a knight's fee have a hauberk, a helmet, a shield, and a lance." Hardly equipment the average peasant would have laying around.

Furthermore, there was some recognition by the Anglo-Norman aristocracy that the peasantry formed the backbone of their economy. While 12th C combatants shunned set-piece battles and large-scale conflicts, most of their warfare was done by ravaging the land, and weakening an enemy's economy. This could mean destroying their fields, disrupting supply chains, burning villages, and killing enemy peasants in order to destroy productivity. Surely the importance placed on disrupting the peasant activity of your enemies means that 12th C combatants would want to protect the productivity of their own.

There was contemporary criticism of these practices, especially the killing of peasants and more grievous acts of destruction. Orderic Vitalis describes the Harrying of the North as being unforgivable, and other monastic chroniclers express shock and disgust at acts they consider excessive. They are especially, and perhaps understandably, critical of the destruction of religious houses and churches. The violent culture of the times was at odds with the religious morals, making these issues all the more complex.

I hope this clears up some misconceptions about military service in the 12th C. And I absolutely welcome historians with knowledge about other regions and time periods to add on!

Sources

Philippe Contamine, War in the Middle Ages (a bit dated, but very comprehensive)

Matthew Strickland, War and Chivalry: The Conduct and Perception of War in England and Normandy, 1066-1217

Matthew Strickland, ed., Anglo-Norman Warfare (This collection of articles has several dedicated to knight service)

Stephen Morillo, Warfare Under the Anglo-Norman Kings 1066-1135

Maurice Keen, Chivalry

Maurice Keen, Medieval Warfare: A History