Hildesheim is a city in Lower Saxony, north-central Germany and has a population of 101,667 (2016). It lies southeast of Hannover on the Innerste River in the foothills of the Harz Mountains. Originally it was a fort on the trade route between Cologne and Magdeburg. Louis I the Pious, son of Charlemagne, founded a bishopric there in 815, an event linked with the thousand-year-old rosebus that blooms above the east choir of the cathedral. Such great prelates as Bernward (bishop 993–1022) and Gotthard (bishop 1022–38) fostered Hildesheim's development as a cultural centre in the 11th century. It became a member of the Hanseatic League and was chartered in 1300. Its bishops were princes of the Holy Roman Empire until 1803, although they lost territory when the town accepted the Reformation in 1542. Hildesheim passed to Prussia in 1803 and then to Hannover in 1815.