 In 1915, Eriksen was also made commander of Agdenes Fortress off Trondheim.In 1931 he reached the rank of Oberst (Colonel) and two years later he obtained the position of commander of Oscarsborg Fortress, a position he would hold until that fateful morning of the Battle of Drøbak Sound on 9 April 1940.By the time of the battle, Eriksen was six months from retirement.