They were small and cheap, and their shallow draft was thought to limit them to defensive operations, which appealed to economy and defence-minded Members of Parliament.

The Admiralty, however, envisioned them attacking shallow-water ports that larger ironclads could not enter and operating in the shallow waters of the Baltic Sea.

They were delivered to the Royal dockyards in 1872 and commissioned for fitting out, but a number of years elapsed before that process was completed, and there was little sense of urgency.

"[4] While not unfit to face heavy weather their decks were frequently awash in even a moderate sea.

Their accommodations were rated the worst in the fleet, referred to by ordinary seamen as "ratholes with tinned air".

Cyclops and Hydra had 4-cylinder inverted compound steam engines made by John Elder that had a working pressure of 60 psi (414 kPa; 4 kgf/cm2).

The engines produced a total of 1,472–1,528 indicated horsepower (1,098–1,139 kW) on sea trials which gave the ships a maximum speed around 11 knots (20 km/h; 13 mph).

Their working pressure was 34 psi (234 kPa; 2 kgf/cm2) and they produced a total of 1,579–1,709 indicated horsepower (1,177–1,274 kW) for about the same speed.

The superstructure and conning tower was fully armoured, the reason it was called a breastwork, with 8–9 inches (203–229 mm) of wrought iron.

This refit also strengthened the breastwork and upper decks, added another watertight bulkhead as well as a false keel.