The earliest evidence of settlements in the area of today's Berlin are remnants of a house foundation dated to 1174, found in excavations in Berlin Mitte, and a wooden beam dated from approximately 1192. The first written records of towns in the area of present-day Berlin date from the late 12th century. Spandau is first mentioned in 1197 and Köpenick in 1209, although these areas did not join Berlin until 1920. The central part of Berlin can be traced back to two towns. Cölln on the Fischerinsel is first mentioned in a 1237 document, and Berlin, across the Spree in what is now called the Nikolaiviertel, is referenced in a document from 1244. 1237 is considered the founding date of the city. The two towns over time formed close economic and social ties, and profited from the staple right on the two important trade routes ''Via Imperii'' and from Bruges to Novgorod. In 1307, they formed an alliance with a common external policy, their internal administrations still being separated.