Project history: These ships were reduced editions of the Powerful class, with a lower speed and a thinner protective deck which saved 3000t displacement and about £100,000 per ship in cost. Andromeda, Diadem, Europa and Niobe were provided under the 1895/96 Estimates and the others under the 1896/97 Estimates. The second group were provided with machinery of higher power for a 0.5kt increase in speed and could be distinguished from the earlier group by the addition of a low wall around the quarterdeck 152mm guns and a bulwark around the after superstructure. All exceeded their designed power and averaged 20.5kts (1st group) and 21kts (2nd group) on full power trials. The completion of Spartiate was delayed by the late arrival of her machinery from the contractors. In general they were good steamers but Niobe suffered from several machinery breakdowns in her early years and boiler problems arose in some ships after a couple of years' service.
    The guns were disposed as in the Powerful except that four single 152mm on shielded mountings were substituted for the 234mm turrets on the forecastle and quarterdeck. They were heavily criticised for their lack of heavy guns and the fact that the guns with the best command were only protected by shields. In other respects they were of similar layout to the Powerful class except that the forecastle deck terminated at the after superstructure.

Ship protection: The protective deck was 102mm thick over the machinery, thinning to 51mm over a narrow section of the crown and reducing to 64mm forward and aft. 12 152mm guns in casemates were protected by 114-51mm armour, four deck guns were protected by 114-51mm shields. Ammunition hoists were protected by 51mm armoured tubes down to the protective deck.

Modernizations: None.

Naval service: Niobe was sold to Canadian Government 6.9.1910 and arrived Halifax 22.10.1910. She ran aground on Cape Sable 29.7.1911 but was raised and repaired. In October 1915 she was disarmed as depot ship, badly damaged by the ammunition explosion in Halifax Narrows 6.12.1917 but continued service until sold in 1922.