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 the 15th century, the city often flipped between being part of the empire and not being part of it. (AUDIO: The Eighth Piece)
Maximilian I was the leader of the Holy Roman Empire in 1504. (AUDIO: The Ghosts of N-Space)
In 1538, Thomas Cromwell noted that he must secure England's position against the Holy Roman Empire. (AUDIO: The Doomsday Chronometer)
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)