Geograph - photograph every grid square

No Man's Land 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 equal largest with Horse Sands Fort. 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 1865 and was completed in March 1880. The cost at the time was £462,500 (equivalent to about £50m in today's money* [2016]).
No Man's Land held 49 guns and barracks for 5 officers and at least 72 men.
It is built of granite, concrete and iron, has a diameter at sea level of about 220' narrowing to 205' at its top. Its main roof rises about 60' above sea level, with the lighthouse adding perhaps another 30' to its highest point.
It is now a luxury (and very unusual) hotel with 22 bedrooms, run by Solent Forts Link
For more detailed technical specification and history see Link
And its Ancient Monument listing with English Heritage Link
It is also Grade II listed Link
One interesting fact about this fort is that in 1971 it was used as one of the locations for the Dr Who story "The Sea Devils" - see Link and Link for more information.
*Note: It is very difficult to compare monetary values over such time periods - it would certainly cost more than £50m 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 £500m these days.