Wikipedia: Birka

Birka was presumably a trading a settlement on the island now known as Björkö, in Lake Mälaren, Sweden. The period of activity in Birka has been estimated from around 700? to the end of the 10th century, thus covering part of the Viking Age.

At its largest, Birka housed about 700 people and its administrative center was supposedly located outside of the settlement itself, on the neaby island of Adelsö. The settlement itself was fortified by a wooden palisade and its harbour? guarded by pilings driven into the bottom of the lake, limiting the number of ships able to pass into it.

In the year 830, the Christian missionary Ansgar visited Birka, an event which is recognized as the first attempt to convert the heathen Swedes.

Birkas role as the trade center of the Swedes was later taken over by Sigtuna.

Today, Björkö is mostly privately owned, and used for farming. The settlement site, however is an archaeological site, and a museum has been built nearby for exhibition of finds, models and reconstructions.