These days Sweden is a peaceful nation, but it’s had a turbulent past with its neighboring countries of Norway, Denmark, and Poland. During periods of fighting it was necessary to alert the Swedish people about the state of the country. This is where the Kastellet citadel comes in.

This small naval fort is located on top of a hill on the small island of Kastellholmen just outside of the old Stockholm city center. Its most distinguishing feature is a forked Swedish flag, flying majestically above the citadel. This flag signifies whether or not the country is at peace. When it’s up all is well in Sweden. But if it’s down or replaced by another country’s flag, that means trouble.

This practice dates back to 1667, when the citadel was first built. It served as a lookout post for incoming ships and a warning to any visitors with ill intent. In 1990, the navy left the fort and attempted to put an end to the tradition, but due to heavy protests from residents of Stockholm, the tradition was picked up again only three days later. Since then a marine is sent to the citadel twice a day, to hoist the flag in the morning and take it down at the end of the day.

The tradition has only been broken twice in the modern era. Once, in 1996, a group of Norwegian expats broke into the building and hoisted up a Norwegian flag for the Norwegian constitution day. Technically, this meant a declaration of war, but the act was quickly resolved as a prank. Another time was in 2008, when, somewhat alarmingly, the flag failed to appear in the morning. However this turned out to be simply because of a broken line on the mast, and fortunately no wars were fought on that day.