Warfare is rarely a pleasant time to be a civilian, and medieval western Europe was no exception. Church councils from the high Middle Ages inveigh against the violent and rapacious secular lords who maraude across their own territory and that of others'. More rarely, they will even convict ecclesiastical lords of similar crimes. In 1049, the Council of Reims charged Bishop Hugh of Langres with armed action, homicides, extortion, and "tyranny" over his clergy. If the Church was charging a Churchman, you know it had to be bad.

The accusation tyrannus, in fact, is all over 11th-12th century sources. Sometimes peasants would even unite to protest against ill treatment by their own lord.

A particularly big fear arising out of petty warfare between nobles was arson. Pope Paschal even went so far as to equate the crime of arson with sacrilege--that's how devastating it could be to property, life, and farmfields.

Siege warfare was another area of frequent and utter brutality. Although Christian scholars did not really start theorizing about "rules of war" until later in the 14th century, the general principle was that any party who remained in a town under siege was automatically an enemy combatant. Even women and children, who were sometimes given the chance to leave the town safely before the siege "officially" began, would earn this label if they stayed. Beyond the hardships of being in a besieged town or castle, the outcome if the fortifications fell was almost certainly death for men; potentially death for women, but also the danger of rape and enslavement.

I've talked before on AH about the sexual violence of warfare in the medieval and early modern world more broadly, so I'll bring over the relevant parts of that answer to this post as well.

In a town or fortification facing a siege, women and children were typically given a choice: flee or stay. Many (most?) chose to stay and help defend their town--we know women were crucial in building and repairing fortifications, and running weapons and provisions to men guarding the walls. (In addition to women who fought directly, like the famous Gesche Meiburg).

But if the town or castle fell, it would be subject to plunder by the enemy soldiers: the seizure of wealth in the churches and monasteries, the execution of men who'd been fighting, the rape and probable execution of the women who stayed. With apologies for straying into early modern, when Protestants captured the town of Pamiers during the French wars of religion, they broke into Catholic families' houses and raped the women they found. The soldiers threw rocks at women and children who fled, trying to stop them.

Plunder was considered the soldiers' right, and this extended to women's bodies. Peter Hagendorf of Bavaria, a rare example of a literate soldier in the earliest modern era, noted in his meticulous diary:

I took a young girl with me from Pforzheim, too, but I let her go back in again. I was sorry about this because at the time I had no wife.

But women outside towns and castles under siege might not fare any better. Villagers forced to house higher-ranking soldiers could well find the nobles demanding sex from their daughters. And those women who did choose to leave towns under siege would, of course, be without protection amidst an enemy army.

Warfare was also a prime source of slaves. On this particular subtopic, most research has concentrated on the Crusades. Nobles in particular had hope that they would be ransomed, and there are a few bright spots of ceremonial use of prisoners-of-war like the role that Arab captives played as honored guests/prisoners at the Byzantine court in the pre-crusader era. But civilian populations as well as the "camp followers" who trailed an army to help with logistics (like finding food and repairing clothing) could generally expect no such quarter.

Christian and Muslim sources alike note several possible outcomes: everyone slaughtered, the old people slaughtered and the healthy adults enslaved; the men slaughtered and women enslaved "because they could always be used to turn the hand mills," as Fulcher of Chartes says archly.

Further reading: A provocative take on the violence of high medieval Europe is Thomas Bisson, The Crisis of the Twelfth Century.