Charlemagne was crowned in Vatican City.
Charlemagne . Charlemagne reached the height of his power in 800 when he was crowned Emperor of the Romans by Pope Leo III on Christmas Day at Old St. Peter 's Basilica . Old St. Peter 's Basilica was the building that stood , from the 4th to 16th centuries , on the spot where the new St. Peter 's Basilica stands today in Vatican City .