What social classes were involved in a power struggle during the Middle Ages?
This sounds like a question about Karl Marx’s idea that all of history is the history of class struggle. Marx says that in every time period of history, there is a struggle between a dominant class and a subordinate class. In Marx’s time, the dominant class was the bourgeoisie and the subordinate class was the proletariat. In the Middle Ages, the dominant class was the feudal lords and the subordinate class was the serfs and/or peasants.
In the Middle Ages, the feudal lords had control of the land. In those days, the land was the source of most of the wealth. The labor was provided by the peasantry, some of whom were unfree serfs. The peasants did the work and the lords (landowners) collected the bulk of the profits from that work. Eventually, the peasants and serfs won the class struggle and the world moved into the era of capitalism in which we now live.