Geograph - photograph every grid square

The Palmerston Fort of Fort Nelson to the north of Portsmouth now houses the big guns of the Royal Armouries National Collection of Artillery.
This is a national museum with free entry, see their website here Link
It houses a vast collection of all forms of artillery from great historical cannons dating back to the C15th to part of the barrel of Saddam Hussein's intended "super gun" with examples of thousands of pieces of artillery from every intervening period.
These are all housed in the numerous rooms and galleries of the old fort, as well as in its large courtyard, and in two hanger-like sheds which contain such things as a vast railway-mounted gun.
A very informative guided tour is available for a modest fee (£3 per person as at 2015) which will point out many of the most interesting features of the site. If you have any interest in military weaponry, you will be able to spend many happy hours in this huge warren!

Fort Nelson was built at considerable expense (£78,649) in the 1860s on the orders of Lord Palmerston, the then Prime Minister. This equates to many millions of pounds in today's money.
It was one of a whole clutch of major fortifications built in this period to defend Britain from the perceived danger of attack from France.
A series of forts, batteries and redoubts were built along the South Coast near to the major Naval bases, especially Portsmouth (as in this case) and Plymouth. The forts were built some way inland from the ports themselves as they were to defend these bases from encircling attacks. Each fort's range overlapped with adjacent forts in order to both create a continuous chain of defence, and so that should one fort fall to the enemy the adjacent fort or forts could bombard it and stop it being used by the enemy themselves.
In many ways the basic design of these forts as exemplified by Fort Nelson was a development of the Star Forts of Henry VIII, but with modern (C19th) innovations and enhancements.
None of these Palmerston Forts ever actually fired a shot in anger and were therefore often seen as expensive "white elephants", but as with all forms of defence they may well have helped to prevent an attack, and therefore served their purpose. During the subsequent world wars of the C20th, the forts continued to play an important role in the defence of the country, though not in the ways that had originally been planned. Fort Nelson in particular was used for various purposes during both these wars, and in WWII held a vast quantity of armaments awaiting use. As such it would have been a primary target for attack by the Luftwaffe, but its strategic importance was carefully downplayed and not advertised by excessive defensive batteries of anti-aircraft guns for example.
For more information on the fort's history see Link
Not surprisingly for such an historic building, it is listed as Grade I by English Heritage - see Link
In particular see Link which has a lot of facts and figures about the fort as well as plans and a virtual "tour".