When the constructions in the period 1886-1894 concentrated on Copenhagen, both Zealand and Jutland were left rather unprotected.
To secure the connection between Falster and Zealand and prevent entry into Roskilde Fjord, some batteries were built.
A battery was built at Lynæs to protect the entrance to the Isefjord. The Lynäs Fort was in use until after the Cold War.
Furthermore, a number of facilities were planned in the warters south of Zealand, including a naval port and a position on the island of Vejrö.
Of these southern facilities only the Grønsund position was built.
It consisted of the Masnedö Fort, The Borgsted Battery and the Haarbölle Battery.