To what extent would peasant levies be used on a medieval (12th century and onwards) western European battlefield? Is there any truth at all to the idea that there would be a core of knights surrounded by a horde of peasants? If yes, to what extent were they used?
Just to clarify, I am not referring to actual, professional soldiers such as English archers, Swiss pikemen or Genoese crossbowmen but rather to something along the Anglo-Saxon fyrd.
For some insight, you may be interested to learn that England in the 13th Century had mandatory requirements for owning armour and weapons based on your wealth. This would include what you might call peasants - freemen with no noble title, although the law specifies all men.
At the high end, the requirement for someone owning fifteen pounds of land and forty marks of goods would be a mail shirt, a helmet, a sword, a dagger, and a horse.
At the low end, with no lower limit, men were required to own a bow and arrow.
So you can see that England essentially had a system whereby, in time of dire need, any man could be expected to be equipped to serve in the military.
I will leave the extent to which that was put into practice to more knowledgeable commentors.
Source: http://www.mediumaevum.com/75years/winchester1285.html