The narrows at Drøbak, called Drøbaksundet, is a natural point for the naval defence of Oslo, the capital of Norway. The first defences were constructed during the reign of Christian IV of Denmark and Norway and were ready in 1644. The fortifications were however not involved in battle during the Hannibal War. After the war the fortifications were dismantled, and only resurrected for a short period during the 1814 war with Sweden.
Around 1830 the discussion started for a renewed fortification of the Drøbak Narrows and the first stage was ready in 1848, the next in 1853. The name of the fortress was given by royal resolution on 23 August 1855 after a visit by the Swedish-Norwegian King Oscar I.
By the end of the 19th century the art of war developed rapidly and the new fortress was soon obsolete. The tension was also growing between the two countries in the union and so the Norwegians decided to upgrade the fortress. From 1890 new improved German guns were installed, an underwater barrier was built in 1874–79, and an underwater torpedo battery was constructed. The main armament was three 28 cm calibre guns manufactured by Krupp. There were also a number of guns with smaller calibres on the mainland. An underwater barrier went from the main islet of Kaholmen and south-west to Hurum on the western side of the fjord, thus making it impossible for large vessels to sail west of the fortress.
Having been constructed in 1898–1901, and taken into service on 15 July 1901, the underground torpedo facility remained one of the few Norwegian defence installations unknown to German military intelligence at the point of the 1940 invasion. The battery was one of two in Norway and it was designed to launch its torpedoes from under the water level. At Oscarsborg the torpedo battery is a concrete construction inside a cave mined into the rock of the North Kaholmen island.
When Norway was invaded on 9 April 1940, all of the fortress' armament was over 40 years old, and of German origin. Both the guns and the torpedo battery worked flawlessly when Oscarsborg encountered one of the German invasion flotillas; they sank the heavy cruiser Blücher, and threw back the German naval force heading for Oslo, thus managing to save the Norwegian King and government from being taken prisoner. The fortress was returned to Norwegian control on 12 May 1945 when Captain Thorleif Unneberg took command of the fortifications and raised the Norwegian flag following the capitulation of all German forces in Norway four days earlier.
During the Cold War Oscarsborg formed a last line of defence for the capital city, with the underground torpedo battery remaining secretly active up until 1 January 1993, having been modernized in the 1980s. After the deactivation of the last weapons systems, the remaining military activity on Oscarsborg consisted of the Coastal Artillery officer training programme. The officer school was officially shut down in 2002. The fortress is now largely a civilian resort and attraction, open for visitors. The scenic surroundings is much used for conferences and excursions. Visitors take a short motor launch trip from Drøbak.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.