Hilsea Lines cover an area of over 80 hectares and features varied wildlife havens including woodland, meadows, hedgerows, marshland and coastal habitats. The area originally served as a military base with the first defences being built in 1544 to protect the navy from an inland attack with further additions in 1757 south of Ports Creek. They were largely unchanged for over 100 years until in 1871 the existing lines replaced them. This was to counter the threat of a French invasion. The two mile structure consisted of chalk and earth ramparts with six bastions and a moat to the north.
Most of the original structure is unaltered. Hilsea Lines were designated a Scheduled Ancient Monument in 1964 and a Conservation Area in 1994. There are interpretation boards across the site to illustrate the significance of the area.
The casemate is a vaulted bombproof building for gun emplacements to provide protection to the guns and their crews. The gun positions were arranged along the front of the casemate and were known as a gun gallery. Troops from the Royal Artillery were responsible for manning the lines from 1860. There were eight 40 pounder Rifled Muzzle Loader Guns at Hilsea Lines during the 1890s and were used to supplement the 150 32 pounder guns.