Cölln was the twin city of Old Berlin (Altberlin) from the 13th century to the 18th century. Cölln was located on the Fisher Island section of Spree Island, opposite Altberlin on the western bank of the River Spree, until the cities were merged by Frederick I of Prussia to form Berlin in 1710. Today, the former site of Cölln is the historic core of the modern Mitte locality of the Berlin-Mitte borough in central Berlin.