In 1987 the Landmark Trust, an architectural restoration charity, recognised the importance of Crownhill Fort and, wishing to give it a secure future, purchased it from the Ministry of Defence. The Trust's intention was not only to ensure the Fort's preservation and to restore its original layout as far as possible, but also to open it to visitors so that they might learn and profit from the experience.
Crownhill Fort, the largest, most advanced, and least altered of Plymouth's 19th century forts, commands one of the highest points in the city yet is surprisingly inconspicuous. Though covering 16 acres and surrounded by a broad, deep ditch hewn from bedrock, the fort appears from only a short distance to be nothing but a forested hilltop. There are, however, four fighting levels with placements for 32 cannons and six mortars, nearly a half mile of tunnels, and accommodation for 300 soldiers and officers concealed within it.
Crownhill Fort was the key to the North-East Defences of Plymouth which stretched from the Tamar River in the west to the Cattewater in the east and included nine other forts and batteries and one keep in between. It was built as part of the largest fortress building boom in British history against the perceived threat of French invasion. There were mutual feelings of fear and distrust between the two nations and after France launched the armoured steam frigate "La Gloire" in 1858, the British Navy's ability to defend the country was seen to be gravely threatened.
Steam power had greatly improved the accuracy and range for artillery. The adoption of explosive shells, combined with ironclad ships reduced the effectiveness of existing defences. The Channel had been an obstacle to sailing ships, but by mid-century was 'nothing more than a river passable by a steam bridge.' In 1859, the Prime Minister, Lord Palmerston, responded to the alarm from a Royal Commission report on the defences of the United Kingdom.
The commission called for a massive fortress building programme to protect dockyards and strategic harbours at an estimated cost of £111,850,000. Parliament reduced the scales of the undertaking but nonetheless by 1867, 76 forts and batteries had been built or were under construction around the principal naval ports of Britain. Over £3,000,000 was spent on the Plymouth defences alone, with Crownhill Fort construction costing £76,000.
Crownhill Fort, along with the rest of the North-East Defences, was designed by Captain (later Major General) Edmund DuCane who also designed Staddon Fort and, with Captain William Crossman, Tregantle Fort. The great advances in military technology enabled them to break from the centuries old practice of continuous line defences. Each of the forts was designed as a polygon surrounded by a ditch which itself was protected by caponiers (powerful, casemated structures which provided flanking fire across the ditch). Guns, sometimes in casemates, lined the tops of the ramparts and the barrack blocks within were made bomb-proof by the use of mounded earth.
From its completion in 1872 until 1986, Crownhill Fort was under continuous military occupation. Various gun pits remain from World War II and the fort was used as an assembly point by forces leaving for the Falklands War.
