The main fortress is built on the basis of the Montalembert principle: a horseshoe-shaped building with vertical granite walls. The fortifications also include three small batteries – the harbour fort and the eastern and western shore batteries.
The main fortress was completed in the 1850s, when it was regarded as one of the most modern fortresses in Northern Europe. Rapid developments took place in relation to artillery, however, and Oscarsborg Fortress was already outdated in 1864.
New construction periods followed. A new torpedo battery was completed in 1901, and a number of outworks, sighting stations etc. were spread over a large geographical area on both sides of the mainland and on nearby islands. The fortifications also include an underwater jetty south of the fortress, which leads all ship traffic along the east side of the fjord.
Architecturally, the fortifications and the buildings are influenced by styles from different periods, since they were built at different times. The oldest defence facilities are early examples of romantic historicism in Norwegian architecture, with extensive use of semi-circular arches, barrel vaults and cross arch vaults.
Another style that can be seen in several places is the Swiss style, for example the Commandant's Quarters at Nordre Kaholmen. The newest buildings consist of barracks and welfare buildings built for education/training purposes by the Armed Forces in the 1980s.
Oscarsborg Fortress has a unique place in Norwegian history because it was the fortress's guns that sank the German flagship Blücher when Norway was attacked on 9 April 1940. The delay this caused made it possible for the Norwegian government to evacuate some important people in Oslo to safety. The King, the Parliament and the Government were thereby able to lead the resistance to the occupation.
Oscarsborg was still in active military use in the second half of the 20th century and up until 2002.