Geograph - photograph every grid square

Spitbank Fort is a circular sea fort built at the behest of Prime Minister Lord Palmerston as one of a series of forts, batteries and other defences intended to protect the south coast of England from attack by Napoleon III (Louis Napoleon)'s France.
It is one of four forts positioned in the Solent and is the second smallest. It was first commenced in July 1861 but work was suspended in Spring the following year due to political wrangling over their effectiveness and suitability. Work recommenced in March 1867 and was completed in June 1878. The cost at the time was £167,300 (equivalent to about £20m in today's money* [2016]), but actually quite reasonable when compared with the Royal William Victualling Yard at Plymouth which had cost £2 million, 60 years earlier.
Spitbank held 15 guns and barracks for 27 during peacetime - this number would have been higher in time of war.
It is built of granite, concrete and iron, has a diameter at sea level of 162' narrowing to 146' at its top.
It is now a luxury (and very unusual) hotel with eight bedrooms, run by Solent Forts Link
For more detailed technical specification and history see Link
And its Ancient Monument listing with English Heritage Link
The other three sea forts in the Solent are:
St Helens Fort (the smallest)
No Man's Land Fort (equal largest, now a luxury hotel)
Horse Sand Fort (equal largest, owned by Solent Forts, intended to become a museum)
*Note: It is very difficult to compare monetary values over such time periods - it would certainly cost more than £20m to build such a fort today due to vastly increased costs, particularly the much higher wages (in real terms) that are paid to workmen now compared to the C19th. I would think such a fort would cost at least £200m these days.