Munkholmen

Munkholmen is an islet in the municipality of Trondheim in Trøndelag county, Norway. The 13,000-square-metre island sits in the Trondheimsfjord about 1.3 kilometres northwest of the island of Brattøra and the mouth of the river Nidelva in the center of the city of Trondheim. The islet has served as a place of execution, a monastery, a fortress, prison, and a World War II anti-aircraft gun station. Today, Munkholmen is a popular tourist attraction and recreation site.
In the years prior to the founding of the city of Trondheim in 997 by King Olav Tryggvason, Munkholmen had been used as an execution site by the Jarls of Lade. The arrival of Olav Tryggvason to Norway in 995 coincided with a revolt against Haakon Sigurdsson, who was killed by Tormod Kark. The severed heads of both Haakon and Kark were placed on stakes on Munkholmen facing out into the fjord to serve as a warning to visitors. The tradition of displaying the severed heads of criminals and political opponents was continued for some time, but the heads were now placed so that they faced the city of Trondheim to deter its citizens from committing crimes.

Wikipedia