This valuable nature area has developed on a site that was originally a military base.
The first defences, built 1544, protected the city and port of Portsmouth from inland attack for 300 years, before the existing Lines replaced them in 1871 when the renewed threat of a French invasion prompted the construction of stronger defences.
This two-mile long structure of chalk and earth ramparts 9m high and 20m wide, has six bastions of bombproof casemates and a moat to the north. The expected invasion never happened and Hilsea Lines have never been used in battle.
Most of the original structure is unaltered today. The Lines were designated a Scheduled Ancient Monument in 1964, and gained Conservation Area status in 1994 in recognition of the value, not only of the monument, but also of its setting.