In the early 17th century the struggle between the Netherlands Navy and the Dunkirk pirates had spread into Norwegian coastline waters in the South of Norway (Sørlandet).
In June 1635 Christian IV of Denmark-Norway, in his vessel which lay at anchor at Flekkerøy harbor, ordered construction of a naval base and Christiansø Fortress on small Slottsholmen island (now called Gammeløen or old island).
The island commands the western approaches to Flekkerøy harbor, a port long frequented by ships of many nations.
While the Dutch-English war (1652–1654) was being fought Jørgen Bjelke was named lord of the region and commander of Christiansø Fortress to strengthen the defenses in the event that Oliver Cromwell's forces should attempt to seize it to prey on the extensive Dutch-Norwegian trade.
When the danger from Cromwell ceased, this fort was abandoned and replaced by the more strategically located Fredriksholm Fortress, which commanded both approaches to the harbor, in 1658.