Birka and Hovgården are archeological sites that are a testimony to the wide-ranging trade network established by the Vikings. Birka was also important as the site of the first known Christian congregation in Sweden, founded in 831 by Saint Ansgar. The town Birka was founded in the 8th century, and was an important harbour until the 10th century AD. It is located on Björkö island. The site includes remains of a fortress, ramparts of a town wall, traces of harbours and thousands of burial mound