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:House of the Blackheads (Melngalvju nams) is a building situated in the old town of Riga. The original building was erected during the first third of the 14th century for the Brotherhood of Blackheads, a guild for unmarried German merchants in Riga. Major works were done in the years 1580 and 1886, adding most of the ornaments.
The structure was bombed to a ruin by the Germans June 28, 1941 and the remains demolished by the Soviets in 1948. The current reconstruction was erected from 1995 to 1999. Today the House of Blackheads serves as a museum and sometimes concert hall.