Which important member of the feudalism system am i? i was hired to protect the kingdom. for my service in the standing army, i received a part of the kingdom's landholdings in an agreement known as subinfeudation.

This is a serf, more commonly known as a knight. Unlike the regular agricultural workers, they didn't have to work in the fields and make produce. They did however have to defend the feudal lord and the people from other feudal lords and their armies. Feudal lords waged wars often so they were na important class in the system and there was a lot of them.