Cölln

Central Berlin in 1688 with Cölln (yellow)

In the 13th century Cölln (German: [ˈkœln]) was the sister town of Old Berlin (Altberlin), located on the southern Spree Island in the Margraviate of Brandenburg. Today the island is located in the historic core of the central Mitte locality of modern Berlin. Its northern peak is known as Museum Island, while the part south of Gertraudenstraße is called Fischerinsel (Fisher Island).

Cölln is first mentioned in a 1237 deed, denoting a priest Symeon of Cölln's (Symeon de Colonia) Saint Peter's Church as a witness. This date is commonly regarded as the origin of Berlin, though Altberlin on the eastern bank of the Spree river was not mentioned before 1244 and part of nowadays Greater Berlin like Spandau and Köpenick are even older.

Cölln and Altberlin were separated only by the river Spree, linked by the Mühlendamm causeway, hence there was a close connection right from the start. Since the trade route from Magdeburg to Frankfurt (Oder) crosses the twin town and the inland water-transportation routes also passed through it, Cölln-Berlin quickly came to prosperity. A second crossing, the Lange Brücke (Long Bridge), today the Rathausbrücke (Town Hall Bridge) was erected across the Spree in 1307 with a common town hall in the middle of it.

The common policy of Berlin and Cölln led 1307 to a first alliance with other towns (Brandenburg an der Havel, Frankfurt (Oder) and Salzwedel) in the March to defend their rights against the sovereign. The Elector Frederick II Irontooth of Brandenburg ended the autonomy of Cölln/Berlin and declared the twin town to his residence in 1451. In 1710 the twin cities Cölln and old Berlin merged by the order of King Frederick I to form the capital of Prussia. As Altberlin was twice as big as Cölln at that time, the merged city was named Berlin.

...
Wikipedia