Designed and built to challenge an aggressive French shipbuilding programme in the late 1850s, when commissioned the 40-gun steam-powered ironclad HMS Warrior was the largest warship in the world. The revolutionary design housed the main guns, 10 boilers and steam engine inside an impregnable armoured 'box' or citadel made from 41/2in thick wrought iron plates. Warrior is the only surviving example of Britain's 'Black Battlefleet' - the 45 iron hulls built for the Royal Navy between 1861 and 1877. She was restored in the 1980s and is now on public display at Portsmouth Historic Dockyard.