The sister town of early Medieval Berlin was Cölln, a settlement on Spree Island. Today this area is in the centre of Berlin near the Museum Island, which is the modern name of the former Spree or Fisher's Island. Originally, this part of the island was a residential complex dedicated to art and science, developed under Prussian King Frederick William IV.

The town of Cölln was recorded before Berlin. It received its first mention in 1237, whereas Berlin was first mentioned in 1244. The river Spree separated the two. Due to their proximity to the trade route connecting Magdeburg to the East they became prosperous very quickly. Especially if one bears in mind that previously Berlin had been a mere fishing village. In addition, Berlin and Cölln united in a common policy, which resulted in an alliance with other towns in order to protect their rights before the supreme ruler. This union was the first one of this kind.

In 1451, the elector Frederick II put an end to the autonomy of the twin towns and declared them a royal residence. However, it was only in 1709 that the towns merged into one, becoming a base of present-day Berlin. The merged city was named after the larger of the two towns. The smaller town's name has survived as New Cölln, a suburb of Berlin. Initially, it was an extension of Cölln to the south.

The first mention of city charters for Berlin and Cölln date back from 1251 and 1261 respectively. Both documents are displayed in the Brandenburg Cathedral's museum. They are actually a part of the very little that is left of these ancient communities. Other remnants can be seen in the Klosterkirche near the Alexanderplatz. Most written records were damaged by the fire in 1830.