 Most of the existing buildings are from the early 20th century, with a few individual buildings which are a century or so older.Birka settlement originated around 800 AD on the shores of the strait separating Björkö and Adelsö, today a pasture-ground known as "Bystan" covering some 12 ha.The area is still delimited by the ancient defensive earthwork, which once must have stretched farther south to the hill fort at "Borgberget".