Stockholm's Kastellet: A 17th-century Citadel on the Islet of Kastellholmen

Kastellet, named after the island Kastellholmen, was established in the 17th century under the orders of Erik Dahlbergh, a soldier who was primarily a builder. The initial citadel, 3 meters high, was deemed insufficient for the city's defense. A taller tower, constructed from red bricks in the 19th century, stands now in its place. The Swedish sky is mirrored from the tower’s top, where it is raised and lowered daily, spring and autumn, signifying peace. Kastellet continues to be a significant part of Stockholm's history and design.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons