Serf 

A peasant who worked on the land. A Serf could not any change his profession or migrate to any other area or marry without the permission of his lord master.

Manor systems

There is Castle in this system where the lord lived. The lord owned large tract of land. The serfs cultivated the land of the lord and the own fields. The serfs paid other dues in cash and king. The lord of the manor-maintained law and order dispensed justice.

(Due to lawlessness, free presents accepted vassalage of the lord of the he manor in return for protection.)

In a feudal system, what mattered was the manner and the extent to which the peasantry could exercise its freedom.

SYSTEM OF MILITARY ORGANISATION

Cavalry warfare replaced infantry. The problem of developing and maintaining the Organization needed for the new mode of warfare helped in the growth of feudalism in Europe.

Army was decentralized, as no king could maintain a large body of cavalry, armor, and equipment out of his resources the fief holders were assigned the responsibility of maintaining fixed forces of Cavalry and infantry for the service of the King.