Kastellet is a small citadel located on the islet Kastellholmen. The first fortification on the location was built in 1667 designed by Erik Dahlbergh. It exploded in June 1845 and subsequently a new was built 1846-1848 to the design of architect Fredrik Blom. It consists of a round tower with red brick walls and a 20 meter high stair tower. On the top flies the Military Ensign of Sweden, it is hoisted and lowered every day, indicating the nation is at peace.
On May 17, 1996, the Norwegian Constitution Day, some Norwegian expats raised the Norwegian flag in the tower. Though such an action would historically have been taken as a declaration of war, a diplomatic crisis could be avoided.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.