Charlemagne was king in 800 ad. Charlemagne was named the Holy leader of his empire. Also, created the feudal system. And he stopped the muslims from advancing into Europe. I think Charlemagne had a positive impact during the time period because he inspired a cultural rebirth in Europe, stopped the muslims from advancing into Europe, and united most of western Europe under one empire.
A castle brought protection during the middle ages because of the moat which could only really be crossed with the drawbridge down. Also, the slits for archers allowed archers in the castle to easily shot out and be protected. And the stone walls were very difficult to knock down.
Feudalism was a system of promises and loyalty. The monarch split up his land and gives it to the nobles. In return the nobles give the monarch taxes, loyalty, and protection. The nobles then give some land to the Knights and in return give the nobles protection, taxes, and loyalty. The knights then give land and protection to the peasants and in return give the knights food, tools, labor, taxes, and loyalty. The Feudal system was developed to better protect the kingdom and I think it was a good system because everyone got something out of it.