Located at the top of a hill on Kastellholmen islet is a classic red brick fortress called The Citadel. When Kastellet was built in 1848, the fort had nine cannons to help defend Stockholm against enemy ships. During World War II, it was fitted with anti-aircraft guns. By 1959, The Citadel was decommissioned. In the 1970s, the former military building was converted into offices. Finally, in 2007, this became the home of the Naval Officers’ Society.