Built on the ruins of a fort built by the French in the late 17th century, this was Gustavia's most important fort during the Swedish period.
It housed a barracks, a stone guardhouse for a dozen men and a wooden hut.
Towards the end of the Swedish period, the battery fell into disrepair, and all that remains of Fort Gustaf are the stone part of the guardhouse and the cisterns behind the weather station, four cannons, two of which are made of fiberglass at the foot of the lighthouse, and two others near the weather station, on loan from the Kariskrona Maritime Museum (Sweden) since December 2011. These cannons recall the days when Gustavia was a fortified city.
In 2004, the weather station was renamed "Espace Météo Caraïbe", offering a meteorological museum space.
In 2015, the weather station became "Espace Gustaf III", now dedicated to the Swedish past and Franco-Swedish friendship.