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In page History of Norway:

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The Neolithic period started in 4,000 BC. The Migration Period caused the first chieftains to take control and hilltop forts to be constructed. From the 8th century Norwegians started expanding across the seas to the British Isles and later Iceland and Greenland. The Viking Age also saw the unification of the country. Christianization was completed during the 11th century and Nidaros became an archdiocese. The population expanded quickly until 1349 (Oslo: 3,000; Bergen: 7,000; Trondheim: 4,000)[citation needed] when it was halved by the Black Death and successive plagues. Bergen became the main trading port, controlled by the Hanseatic League. Norway entered the Kalmar Union with Denmark and Sweden in 1397.