The historic Viking trading centre of Birka, on Björkö in Lake Mälaren, makes a fantastic day trip. A Unesco World Heritage site, it was founded around AD 760 to expand and control trade in the region. The village attracted merchants and craft workers, and the population quickly grew to about 700. A large defensive fort with thick dry-stone ramparts was constructed nearby. Birka was abandoned in the late 10th century when Sigtuna took over the role of commercial centre.