Kastellet, Military citadel on Kastellholmen islet, Stockholm, Sweden

Kastellet is a round tower with red brick walls and two crenellated wing buildings on a high grey stone base, standing on the islet Kastellholmen. A 20-metre-high round stair tower holds a spiral staircase built without a central post. The wings, in red pressed brick, once contained barracks quarters; the building was raised between 1846 and 1848 to designs by Fredrik Blom.

The fortress was built in 1667 following a design by Erik Dahlbergh and was originally created to defend the city of Stockholm. After an explosion in 1845, it was completely rebuilt between 1846 and 1848 and has retained its current form since then.

Since 6 June 1990, two guards from the Stockholm Palace main guard march to Kastellholmen each day to raise and lower the Swedish Military Ensign, the three-tongued flag flown here since the 1660s. When the Coastal Artillery school left in 1990 the flag was hauled down for good, until the commander of HMS Stockholm had sailors hoist it again as his ship departed on 2 April 1990.

The fortress is accessible on foot by crossing a bridge or causeway and can be easily reached from the old town area. Visitors should be prepared for weather conditions since the exposed location on the water offers less shelter.

On 17 May 1996, Norwegian Constitution Day, Norwegian expatriates in Stockholm briefly raised the Norwegian flag on the tower. The daily hoisting of the Swedish military ensign had resumed in 1990 after the commander of HMS Stockholm sent sailors ashore to raise it in protest when the coastal artillery school left.