Fredriksborgs fästning, Värmdö Island

Fredriksborg Fortress is a fortification in the municipality of Värmdö at Oxdjupet, the entrance to Stockholm. The fortress consisted of towers, fortifications and surrounding buildings. The fortress was part of the defences built in the Stockholm archipelago to protect Stockholm after the Russian sieges along the east coast in 1719.

The fortifications, completed as early as 1735, were named Fredriksborg Fortress after King Frederick I and were considered one of the most modern of their kind in Europe. During the 19th century, Fredriksborg fell into disrepair and the fortress was replaced by Vaxholm Castle. Opposite Fredriksborg Fortress, Oskar-Fredriksborg was built between 1870 and 1877. The buildings of Fredriksborg Fortress are managed by the National Property Board and have been a protected monument since 1935.