Hilsea Lines is a popular attraction for both residents and visitors as a green corridor that separates Portsea Island from the mainland. Covering an area of over 80 Hectares, it is one of the most varied wildlife havens in Portsmouth, containing woodland, hedgerows, meadows, both fresh and brackish water areas, marshland and coastal habitats.

This valuable nature area has developed on a site that originally served as a military base. The first defences were built in 1544 to protect the naval interests from an inland attack and further established in 1757 south of Ports Creek. They remained largely unaltered for over 100 years before the existing Lines replaced them in 1871 when the renewed threat of a French invasion prompted the need for stronger defences. This 2 mile long structure consisted of chalk and earth ramparts 9m high and 20m wide, with six Bastions of bombproof casemates and a moat to the north. The expected invasion did not materialise and the Lines were never used in battle. Some changes took place early 20th Century, but most of the original structure is unaltered. The Lines were designated a Scheduled Ancient Monument in 1964 and the site was given Conservation Area status in 1994 in recognition of the value, not only of the monument, but also of its setting