Fort Nelson is a polygonal fort and was built as a result of the 1859 Royal Commission by Lord Palmerston. Built between 1861 and 1870 it mounted up to 58 guns, primarily 7-inch RBL. It provided land defence to Portsmouth Dockyard, and is one of five forts built on the Portsdown ridge line. It was an advance position to replace the old Hilsea Lines which lie some eight miles to the south. The defence line included:
- Fort Fareham
- Fort Walllington
- Fort Nelson
- Fort Southwick
- Fort Widley
- Fort Purbrook
- Crookhorn Redoubt
- Farlington Redoubt
Fort Nelson is now home to Royal Armouries.