The second line of defence included the older parts of the fortress and contained most of the storage and supply areas and other rear line functions. The fighting power of the second line was limited to quick-firing 75 mm and 57 mm guns and old 11 inch cannons and mortars. The primary combat objective of the batteries on the second line was to prevent enemy ships from passing through Kustaanmiekansalmi or Särkänsalmi straits to the inner anchorage on Kruunuvuorenselkä. The second line included Lauttasaari, Melkki, Pihlajasaaret, Harakka, the central citadel at Suomenlinna, Lonna, Vallisaari, Kuninkaansaari, Nuottasaari and Vasikkasaari.
