The site of a probable Neolithic causewayed enclosure on Halnaker Hill, partly surviving as an earthwork. A later post mill and World War II searchlight emplacement are also located on the hill. Halnaker Hill is an isolated chalk bluff which projects to the south from a ridge of the Sussex Downs and commands panoramic views over the surrounding landscape, including the Long Down flint mines.
It consists of a single causewayed circuit, of partially angular layout, with an inturned entrance in the south. Three further probable and two possible entrances or causeways can be observed either on the ground or on air photographs. Some of it survives as a slight earthwork enclosing some 1.7 hectares. It measures approximately 175 metres by 145 metres, with its long axis aligned south-south-west to north-north-east. The enclosure originally comprised a fairly continuous low bank and external ditch, but it has suffered severely as a result of modern ploughing. The bank survives up to 5 metres wide and 0.5 metres high. Only very slight traces of the ditch are visible on the ground, with a maximum width of 4 metres and depth of 0.1 metres.
Investigations between 1981-1983 led to the discovery of fragments of pottery within the ditch, some of which have been dated to the Neolithic period. Late Bronze Age and Roman sherds were also found. The later post mill lies close to the southern entrance of the earlier enclosure and survives as a roughly circular mound about 19 metres in diameter and up to 0.8 metres high, with a large central depression. Historical records suggest that the windmill was sited here in 1540 by the Duke of Richmond in order to operate as the feudal mill for the Goodwood estates. During World War II four large searchlights were used on this hill to seek out raiding enemy aircraft. The searchlight emplacements, the middle two of which are situated within the earlier enclosure, were constructed about 100 metres apart in a south west facing, semicircular arc.