The forts on Portsdown Hill were built as a result of the 1859 Royal Commission, as part of a series of fortifications built to defend Portsmouth and its dockyard (which is 8 kilometres away) from a possible attack from inland, as the development of rifled gun barrels made it possible for an invading army to land elsewhere, circle around to the top of the hill and bombard the city from there, rendering the existing Hilsea Lines at the bottom of the ridge useless. A series of 6 forts were planned along the of the ridge. From west to east they are forts Fareham, Wallington, Nelson, Southwick, Widley and Purbrook. The line was finished off at the eastern end with Crookhorn Redoubt and Farlington Redoubt. The main threat was perceived to be from Napoleon III of France, but this receded soon after the forts' completion. Due to this they became known as Palmerston's Folly but, though never needed for their original purpose, were useful anti-aircraft gun emplacements during World War II.