isosaari is the largest of the outlying islands. the island's central location and size made it the most heavily armed coastal fort and it was part of the plans for expanding krepost sveaborg already in 1909. in may 1914 construction of four concrete fortified batteries began: one 10 inch battery, one 11 inch battery and two 6 inch batteries. the start of the war hastened the consctruction works that had progressed slowly, and three 10 inch guns were placed on temporary wooden platforms. the concrete batteries were finished by autumn 1915, but a change in plans resulted in the second 6-inch battery being located in santahamina instead. in january 1916 work began to modify the 11-inch battery (battery number 2) for 10-inch guns, with the modifications complete in 1916. the first 10-inch battery is located on the south-western tip of the island, while the other two batteries are on the south-eastern shore. a 150 cm searchlight was located near the first 10-inch battery and a 200 cm searchlight near the two other batteries. a narrow gauge military railway was built on the island to transport supplies and ammunition.