The Hilsea Lines were built to defend Portsmouth Dockyard from land attack.
The existing Hilsea Lines are the third evolution of fortifications to be built on this site. The previous two defence lines had been built in the 16th and 19th Centuries respectively. There are some World War I and II additions such as a pillbox and gun emplacement.
In the 1850s Colonel W.F.D. Jervois, then Assistant Inspector General of Fortifications recommended the Napoleonic era lines be retained with additional casemates to hold artillery pieces to provide flanking fire. The lines are of packed earth with a continuous walkway (ramparts) along the top. The whole length was protected by a wet moat. Ramps were built to allow 40-pdr RBL guns to be mounted along the Lines when required. The gun galleries mounted 7-inch RBL guns firing through small embrasures. The rear of the gun platform, at a lower lever, are casemated barracks. Expense Magazines are distributed along the Lines, with two large feeder magazines.
The lines including the following major features:
- Casemates about 60 metres long with separate front a rear features:
- A gun gallery with 7-inch RBL guns firing through embrasures protected by merlons
- Casemated barracks, each for five men, with access from the rear of the lines
- Each section of casemates had a magazine and gun store
- A sally port located under the West Central Curtain with a steel loop holed door
- Expense Magazines distributed across the Lines
- Tw0 large Magazines to supplement the expense magazines
- A gateway on the West Curtain which has was demolished in 1920
- Five Moncrieff Mountings for 7-inch RBL disappearing guns. Installed in 1875