Fort Drum was constructed on El Fraile Island beginning in 1909 and completed in 1914. El Fraile Island was originally a Mine Control Station and a mine casemate. Fort Drum is about 7500 yards south of Caballo Island and was the most unique of the Harbor Defense forts. On the original island as a base, army engineers completed in 1914 a reinforced concrete battleship 350 feet long by 144 feet wide, the top deck being 40 feet above mean low water and 20 feet thick. Exterior walls were approximately 25 feet to 36 feet thick. Equipped with 4-14 inch M1909 Rifles in armored turrets that had the capability of shooting a 1,660 pound armor piercing shell to a range of about 19,000 yards, 4-6 inch casemated guns and 2-3 inch AA guns. There was a 60 foot fire control cage mast. The fort was considered impregnable to enemy attack.