Hammershus Castle was a center of power on Bornholm in several periods from the 12th century until the 18th century when it was finally abandoned. Hammershus played a central role in the centuries long battle between church and state.
It is not known when Hammershus was built and by whom, but it is known to have played a part in the power struggle between King Valdemar Sejr Of Denmark and Archbishop Anders Sunesen of Lund, and the construction of Hammershus was probably part of an agreement reached at the end of their fight.
But peace never lasted. In 1149 another peace agreement was reached between the Danish King Svend Grathe and Swedish Archbishop Eskild af Lund, in which the Archbishop was given control of three of the four districts on Bornholm including Hammershus. Instead the king built Lilleborg in Almindingen in the remaining district.
But the three districts were not enough for the Swedish archbishop, and in 1259 Lilleborg was sacked by Swedish and German soldiers led by Archbishop Jakob Erlandsen's brother and Duke Jaromar of Rügen. All of Bornholm was now under the archbishop. This was not without problems - the Pope wrote a letter reprimanding the Archbishop for what had happened, and the ferocity with which it was done.
In the next 100 years Hammershus was captured by the Danish king several times and eventually handed back to the Swedish archbishop again every time. However in 1362 King Valdemar Atterdag handed Hammershus back with the condition that Hammershus was to be given back to the Danish crown when it asked for it. This happened in 1522 by request of King Christian II.
Hammershus is built as a typical midieval fortress with several defensive rings around the central castle compound. Taking the castle was not just getting past one, but three or four walls. Hammershus was basically a treasure chamber for whoever was in control of the castle and Bornholm. Taxes were collected in the form of butter, meat, grain and livestock and all of it was collected at Hammershus where part of it was consumed and the rest was shipped to the Archbishop in Lund.
In the 16th century the Hanseatic powers in the Baltic grew very powerful, and especially forces from Lübeck attacked Bornholm and Hammershus several times. After an agreement with the Danish king they took control of Hammershus and Bornholm in 1525 and kept control until 1576. During this time they expanded Hammershus considerably. When Hammershus and Bornholm came back into Danish hands, the castle was neglected and never again reached the importance it once held. The Swedes attacked and held Hammershus for a period until the Bornholm population rose and threw them out, and in 1743 Hammershus was finally abandoned. Today the castle is the largest ruin in Northern Europe.