The Grönsund Stronghold was constructed in the waters south of Zealand.
It consisted af one coastal fort and two coastal batteries.
The Grönsund stronghold
Of the planned positions at the waters of this area, only the Gronsund position was built.
It which consisted of the Masnedö Fortet, the Borgsted Battery and the Haarbölle Battery.
The first at the island og Masnedö and the last two batteries located on the southwestern part of the island of Mön.
The other planned facilities, The Vejrö Fort and the Helholm Battery were never built, just as a planned naval base at the strait of Masnedsund and the Masnedö was never implemented.