the central citadel of krepost sveaborg was formed on the old swedish fortress of sveaborg, modern suomenlinna . it included the islands of kustaanmiekka, susisaari, iso mustasaari, pikku mustasaari, länsi-mustasaari, särkkä and the skerry of pormestarinluoto. except for the smaller särkkä and pormestarinluoto the islands were connected by bridges or joined together. the islands housed the fortress commander and headquarters along with the main telephone exchange and the largest dry dock in finland at the time. the citadel also had weapons and ammunition depots and other storage areas. most of the fortification works were concentrated on other islands, and of the older armament only one battery of three 11 inch guns remained in active service in kustaanmiekka island. to guard kustaanmiekansalmi strait two batteries of 57 mm guns were built, one on kustaanmiekka and one on iso-mustasaari island. a third battery of 57 mm guns on länsi-mustasaari guarded särkänsalmi strait.