Aachen Cathedral, Germany, Interior. Frequently referred to as the "Imperial Cathedral" (in German: Kaiserdom), is a Roman Catholic church in Aachen, Germany. The church is the oldest cathedral in northern Europe and was known as the "Royal Church of St. Mary at Aachen" during the Middle Ages. For 600 years, from 936 to 1531, the Aachen chapel was the church of coronation for 30 German kings and 12 queens. The church is the episcopal seat of the Diocese of Aachen.