 "Formidable" had a main battery of four 40-calibre guns mounted in twin-gun turrets fore and aft; these guns were mounted in circular barbettes that allowed all-around loading or elevation.The ships also mounted a secondary battery of twelve 45-calibre guns mounted in casemates, in addition to ten 12-pounder guns and six 3-pounder guns for defence against torpedo boats.As was customary for battleships of the period, she was also equipped with four torpedo tubes submerged in the hull.