Bornholm has a long history as a military and political center in the Baltic ocean. The history goes back to the famous Vikings. Numerous findings at southern Bornholm indicate that Bornholm was a well established trading point. Poland, Sweden and Germany are all in the range of 100km. That made Bornholm attractive for the Danish king and the archbishop of Lund and the city of Lübeck during the medieval. Catholics who fasted were only allowed to eat fish and other similar things, so that in the 13th century herring was the main income source on Bornholm. http://wikitravel.org/en/Bornholm