The Longis Lines was a stretch of parapet wall that covered Longis Bay. The defence line included a parapet wall and fourteen gun emplacements with magazines. Most of the guns were on garrison carriages, but at least two were on traversing platforms by 1840.
In 1842 the Longis Lines were armed with two 20-pdr guns, six 18-pdr guns and two 32-pdr carronades. Fort Ile de Raz on Rat Island was built on the Northern approaches and protected the opposite side of the bay.
Plans from 1842 show three Batteries along the Longis Lines with a total of ten guns (TNA MPH 1/868). Number 1 battery had two guns on traversing platforms, while Number 2 and 3 Batteries had two guns on garrison carriages firing through embrasures. while Number 3 Battery. The guns were two 20-pdr, six 18-pdr and two 32-pdr SB pieces. The There was a breast works to the west, with a magazine built here.
By 1852 the Number 1 to Number 4 Batteries had been renamed A, B, C and D Batteries respectively. On the West end of the Lines there was a new gun on a traversing platform with all round fire, names Frying Pan Battery (MFQ 1.1298).