Charlemagne's kingdom became he Holy Roman Empire because he wanted to have all the Germanic tribes to be one kingdom.
Charlemagne wanted all of his people to read and write so he could give us notes on stuff to do and we could respond and know what to do.
I am one of Charlemagne's scholars, and he gave me the important job of copying all the old manuscripts into Latin, BY HAND! it is really important because the manuscripts have important knowledge, and if I copy them, I save the knowledge.
Charlemagne was responsible for the Christian Europe because he converted all the people he conquered, and killed all the people that didn't convert.
The changes Charlemagne had to the legal system was that instead of trial by ordeal and having to fight, you went in front of honorable men and they had to judge fairly, and that determined if you were innocent.
How Charlemagne was "great" was because of how he improved his kingdom. One example of how Charlemagne improved his kingdom was that he had silver as money and had it so that the same thing was the same amount of money.