The Grönsund stronghold
The Grönsund stronghold was built according to the political 1909 defense agreement as part of a larger overall plan for the defense of Sealand.
The stonghold was constructed over the years 1912 – 1915.
The Grönsund stronghold should protect the southern waters of Zealand.
It consisted of the Masnedö Fort ,the Borgsted Battery and the Haarbölle Battery.