The Birka archaeological site is located on Bjorko Island in Lake Malar and was occupied in the 9th and 10th centuries. Hovgarden is situated on the neighbouring island of Adelso. Together, they make up an archaeological complex which illustrates the elaborate trading networks of Viking-Age Europe and their influence on the subsequent history of Scandinavia. Birka was also important as the site of the first Christian congregation in Sweden, founded in 831 by St Ansgar.