Complex sentence: The chapter had the right to elect independently the bishop and resided at Braunsberg (Braniewo) until it moved to Frauenburg (Frombork) in 1280 after attacks by heathen Old Prussians.
Split into simpler sentences: The chapter had the right to elect independently the bishop. It resided at Braunsberg (Braniewo) until it moved to Frauenburg (Frombork) in 1280 after attacks by heathen Old Prussians.