The Bishopric of Brandenburg was a Roman Catholic diocese established by Otto the Great in 948, including the territory between the Elbe on the west, the Oder on the east, and the Schwarze Elster on the south, and taking in the Uckermark to the north. Its seat was Brandenburg upon Havel. It was a state of the Holy Roman Empire for some time, probably starting at 1161/1165, but never managed to gain control over a significant territory, being overshadowed by the Margraviate of Brandenburg, which was originally seated in the same city. Chapter and cathedral, surrounded by further ecclesiastical institutions, were located on Dominsel (cathedral island), which formed a prince-episcopal immunity district, distinct from the city of Brandenburg.