As the Royal Navy's main base in the 19th century, Portsmouth needed to be the most heavily defended when threatened by French aggression and advances in their ships designs in the mid 1850's. The Royal Commission set up in 1859 proposed numerous defences be constructed to supplement the old, now inneffective, fortifications.

Portsmouth was to be protected by a line of forts and gun batteries along the coast from Fort Gilkicker to Fort Cumberland and two chains of land forts to protect the harbour from an invasion force attacking from the north or east by land. The five Gosport Forts (Forts Elson, Brockhurst, Rowner, Grange and Gomer) protected the west side of the harbour and the five Portsdown forts (Forts Purbrook, Widley, Southwick, Nelson and Wallington), built between 1861 and 1874 on the top of Portsdown Hill, were to protect the north and east approaches. The two lines of forts were linked by Fort Fareham. They all have a low profile and are disguised by earth and grass banks on the western or northern sides respectively.