The Viking Age settlements Birka on the island of Björkö and Hovgården on the neighbouring island Adelsö have been an UNESCO World Heritage Site since 1993 , as has Drottningholm Palace on the island of Lovön .
The Viking settlements Birka on the island of Björkö and Hovgården on the neighbouring island Adelsö have been a UNESCO World Heritage Site since 1993 , as has Drottningholm Palace on the island of Lovön .