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. Fort Fareham is now a small industrial estate. The remains of the Fort are Grade II listed. Fort Wallington has been largely demolished to make way for an industrial estate. What remains is now a Grade II Listed Building. Fort Nelson has been extensively restored as an artillery museum run by the Royal Armouries. It is a Grade I Listed Building, the highest level of designation. Fort Southwick was for many years an Admiralty Research Establishment, sold in 2003 for housing. It is also now a Grade I Listed Building. Fort Widley is owned by Portsmouth council, hosting a stable and various community rooms. Tours run on summer weekends. It is Grade II* Listed. Fort Purbrook is open occasionally for craft fairs. It is also home to an activity centre which offers (but is not limited to) archery, rifle shooting and indoor rock-climbing. It is also Grade II* Listed. In October 2021, the fort was one of 142 sites across England to receive part of a £35-million injection into the government's Culture Recovery Fund. Guarding Fort Purbrook, the Crookhorn redoubt suffered from subsidence, and was demolished by 1876. The Farlington redoubt had only the ditches and gun positions dug, and was finally demolished in the 1970s. The tunnel between Crookhorn and Purbrook has been rediscovered. Portsdown also gave its name to an army estate in Singapore. Long after the British army moved out (1967) the Portsdown estate continues to thrive, with Portsdown Road the main road running through the middle. Fort Widley Fort Purbrook Fort Nelson Portsdown SSSI Over fifty hectares of the south face of the hill are a Site of Special Scientific Interest owing to its chalk grassland habitat. Grazing ceased in the