The narrows at Drøbak, called Drøbaksundet, is a natural point for the naval defence of Oslo, the capital of Norway.

[3] The underwater barrier extended from the main islet of Søndre Kaholmen southwest to Hurum on the western side of the fjord, thus making it impossible for large vessels to sail west of the fortress.

[5] This torpedo facility remained one of the few Norwegian defence installations unknown to German military intelligence at the time of their 9 April 1940 invasion.

[4] At Oscarsborg the torpedo battery is a concrete construction inside a cave mined into the rock of Nordre Kaholmen island.

Under the command of Colonel Birger Eriksen, the fortress' armaments, which had been supplied by the German Empire more than 40 years before, worked flawlessly against the invasion fleet.

Oscarsborg's stoic defense with antiquated weapons threw back the German naval force heading for Oslo.

[1] Although the German naval attack on Oslo had been thwarted by the actions of Oscarsborg, the city was seized later that day by forces that were airlifted into Fornebu Airport.

In light of the fall of the capital, and with news of German landings at the village of Son south of Drøbak,[9] Colonel Eriksen decided that further fighting without adequate infantry support was in vain, and agreed to a ceasefire on the evening of 9 April.

Three days later on 12 May 1945, Oscarsborg fortress was returned to Norwegian control when Captain Thorleif Unneberg took command of the fortifications.

[10] During the Cold War Oscarsborg formed a last line of defence for Oslo, with the underground torpedo battery remaining secretly active up until 1 January 1993,[1] having been modernized in the 1980s.

[5] After the deactivation of the last weapon systems, the remaining military activity on Oscarsborg consisted of the Coastal Artillery officer training programme.

[13][14] On 8 April 2014 the 28 cm guns of the main battery were fired with blank rounds when the fortress was used as a location of a scene for the production of the 2016 film The King's Choice, directed by Erik Poppe.