Trekroner Søfort (literally Three Crowns Sea Fortress) is a sea fort at the entrance to the Copenhagen harbour. From 1713 until after World War I, Trekroner Fort was part of the fortifications of Copenhagen. The original location of Trekroner Fort was a few hundred meters north of the current one. In 1713, three old ships of the line were sunk to form the basis for a battery. One of the ships was called Trekroner, and she gave her name to the fort.
Construction of the current fort began in 1787. The fort was an important part of the Danish line of defense during the Battle of Copenhagen in 1801. The fort also was engaged during the British attack on Copenhagen in 1807. From 1818 to 1828 and in 1860, the fort was strongly enhanced, but its military significance diminished after the First World War. In 1934 it was sold to the Copenhagen harbour services. During the German occupation of Denmark the Germans used the fortress as a barracks. After the war it was used for a few years as a prison for German officers and the former German ambassador to Denmark. Afterwards, it fell vacant until 1984, when it was opened to the public. Trekroner Fort is one of three artificial islands that were created to defend the entrance to Copenhagen’s harbor. (The other two are Middelgrundsfortet and Flakfortet.)
Trekroner Sea Fortress is a small, artificial island with two distinctive red barracks, situated at the entrance to Copenhagen Harbour. It was originally part of the Copenhagen Fortifications. Today, visitors have access during summer, allowing them to explore the old gun casements and gloomy, historic cellars, and to enjoy the view of the Copenhagen skyline. The oldest part of the fortress dates back to the late 18th century. Only after the defeat by the Germans in 1864 was it reinforced with the solid casements, ammunition storage and gun batteries. It was decommissioned as a naval fortress in 1932. In the years leading up to the Second World War, it was a favourite summer attraction, with a restaurant, cabaret, theatre and other attractions.
The fort is framed by piles and a strong frame structure, that was filled with soil and mud from Copenhagen as well as a deepening of the harbor. As early as 1801, before the fort was completed, it took part against the English in the Battle of the Rhed. The fort was hastily equipped with 66 guns and 660 men, and together with the navy’s ships and some hastily equipped naval batteries the enemy, with the admirals Parker and Nelson at the head, was kept away from Copenhagen. This was, however, not enough to avoid defeat. Again in 1807, Trekroner was active during the English bombardement of Copenhagen.
After the state bankruptcy, there were no funds for the neigher the Army nor the navy, and it was not until 1818 that the king raised the funds for the completion of the Trekroner. It was completed in 1828. The bulwarks were now reinforced with a 3 meter thick sandstone wall covered with granite. In the throat (the backside of the fort), shooting openings were made. In 1838-39 the two barracks buildings were erected at the entrance to the harbor of the fort. During the Danish-Prussian Schleswig wars, the fort was manned by 390 men. During the period 1865-1869, the fort was expanded with the casemate building, batteries with magazines built into the traverses and a mine control station.
Vestvolden.info: The Fortifications of Copenhagen 1886-1920