En date dk Skanderborg
As the fishing industry thrived in Copenhagen, particularly in the trade of herring, the city began expanding to the north of Slotsholmen.
With the establishment of the Kalmar Union (1397–1523) between Denmark, Norway and Sweden, by about 1416 Copenhagen had emerged as the capital of Denmark when Eric of Pomerania moved his seat to Copenhagen Castle.
Substantial discoveries of flint tools in the area provide evidence of human settlements dating to the Stone Age.
The earliest written mention of the town was in the 12th century when Saxo Grammaticus in Gesta Danorum referred to it as Portus Mercatorum, meaning Merchants' Harbour or, in the Danish of the time, Købmannahavn.
Beginning in the 17th century it consolidated its position as a regional centre of power with its institutions, defences and armed forces.The name of the city reflects its origin as a harbour and a place of commerce.The original designation, from which the contemporary Danish name derives, was Køpmannæhafn, meaning "merchants' harbour", often simply Hafn or Havn ("harbour").The Copenhagen Metro launched in 2002 serves central Copenhagen while the Copenhagen S-train and Lokaltog (private railway) and the Coast Line network serves and connects central Copenhagen to outlying boroughs.To relieve traffic congestion, which is partly the result of increased traffic because of the Fehmarn Belt Fixed Link road and rail construction is planned because the narrow 10 mile isthmus between Roskilde Fjord and Køge Bugt (Køge Bay) forms a traffic bottleneck.