Nazi Germany invaded and occupied Norway in 1940. After capturing Trondheim early on in the Norwegian Campaign, the Germans quickly established a submarine base, exploiting the natural protection provided by the fjord. At this time, Munkholmen was fitted with anti-aircraft weaponry. A large portion of the fort was retrofitted to hold ammunition, and the flooring planks were nailed in with wooden nails to prevent explosions caused by soldiers' boots striking metal nails. The German occupying forces remained in Norway until the end of the war in Europe in May 1945. Remnants of the installation still exist in the upper levels of the fort.