The island has also been used for military installations, and former military buildings and artillery remain on the island. When Denmark-Norway became involved in the Napoleonic war, two cannon batteries were built to defend Oslo and Akershus Fortress in 1808 (again using the monastery ruins as a quarry). Four depots for gunpowder were built in 1826 on the high points of the island to ward off intruders, and a fifth was built in 1867. On the eastern half of the island is a Victorian house, popularly known as the "Laboratory". It was used by Director of Armory Ole Herman Johannes Krag, co-inventor of the Krag–Jørgensen rifle in the late 19th century.