Holy Roman Empire

The Holy Roman Empire was an empire which existed in Europe. Pope Leo III crowned Charlemagne as the leader of the Holy Roman Empire. (PROSE: The Lonely Computer)

At some point before the 1980s, Prague was the capital of the Empire. (PROSE: Leap Second)

In an alternate timeline in which all of history occurred at once, Winston Churchill was the Holy Roman Emperor. (TV: The Wedding of River Song)