What is "munkholmen"

Munkholmen is an islet north of Trondheim, Norway. It sits in the Trondheimsfjord about northwest of the island of Brattøra and the mouth of the river Nidelva. 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.