Fort Grosnez was built between 1850 and 1853 and was the first of fortifications to be completed to protect the Port of Refuge. The Fort is constructed of dressed stone, some sandstone and some granite. Fort Gosnez was position to help protect the harbour from both land and naval attack. There is a prominent south facing glacis covered by No.1 Battery. The dry moat on this face is covered by a caponier. The barracks at Fort Grosnez could accommodate 112 men.
In total Fort Goznez had twenty eight guns mounted in seven batteries giving fields of fire all round. The batteries were: