The middle ages lasted from the 5th century to the 15th century.
I believe Charlemagne that Charlemagne was a good king/emperor because he was a good military strategist. Another reason he was a great leader was because he ensured the survival of Christianity. The final reason he is a great leader is because he was a talented diplomat.
Medieval Castles
Medieval castles protected inhabitants with a moat which people had to swim through also archers could just be waiting for you and shooting at you also people could dump hot oil on you burning you to death. The walls were smooth so you couldn't climb up and archers were almost impossible to hit by hiding behind the bricks when they're not shooting.
My Viking was names Gandalf which is actually a real Norse name.
Vikings affected feudalism by causing the knights and serfs to fight and when the serfs where fighting they could not farm and grow food for the nobles which would make the civilization low on food causing it to be rationed.