Five miles inland on the crest of Portsdown Hill, one of the chain of Victorian forts built to defend Portsmouth from a feared French invasion. Fort Nelson now houses part of the Royal Armouries' artillery collection: cannons, guns, and missile launchers from the 14th century to the Cold War, displayed in the bare brick galleries and outdoor parade grounds of the fort.
The location has the additional virtue of one of the best views in southern England — Portsmouth, the dockyard, the Solent, and the Isle of Wight all spread out below you. Free entry. Best reached by car; bus 39 runs from Portsmouth city centre.