Siarö Fort was a coastal artillery fort, built to protect the approaches to Stockholm from hostile seaborne forces. To protect it from sight and attack, it was constructed entirely underground, apart from the gun emplacements themselves and construction started in 1916.
The fort was equipped with two 15cm guns, later four 57mm guns were installed. The fort also had two searchlight positions. Underground, there is a main spine corridor with the barracks, plant rooms, magazines and other services leading off to either side.
The fort was plagued by dampness throughout its life and its operational days eneded in 1939. It remained in military hands for training purposes for a further 20 years and then lay derelict. Volunteers tidied the site up and it opened as a small museum in the late 1990s. Today the small island has visitor accommodation and the fort is opened daily in summer months and at other times by arrangement.