The Siarö fort was built between 1916-1929. It was the largest of the forts in the outer line. The main task of the fort was to block the Furusund shipping lane in Stockholms northern Archipelago. The fort was armed with two 15cm guns in armor turrets an one smaller battery of two 75mm guns. These smaller guns were later changed to two 57mm guns. Ten officers, 6 NCOs and 250 soldiers were stationed here.