The main armament of the "Powerful"-class cruisers consisted of two Mk VIII guns in single gun turrets, one each fore and aft of the superstructure. Her secondary armament of a dozen Mk I or II guns was arranged in casemates amidships. The end casemates were the first two-storey (guns on the main and upper decks) casemates in the RN. For defence against torpedo boats, sixteen 12-pounder 12-cwt guns and a dozen 3-pounder () Hotchkiss guns were fitted. Two additional 12-pounder 8-cwt guns could be dismounted for service ashore. The ships also mounted four submerged 18-inch (450 mm) torpedo tubes, a pair on each broadside.