Directly across the English Channel from France, about a mile from Portsmouth, they were built about 150 years ago at the order of the prime minister at the time, Lord Henry Palmerston. He feared a French invasion after Napoleon III -- nephew of the much more famous Napoleon Bonaparte -- went from popularly elected French president to emperor of France through a coup.
Where once there were soldiers, now there are weddings, day spas and highbrow vacations. A company called AmaZing Venues has been snapping up these little defensive rotundas and transforming them into luxury accommodations, similar to those you might find on a cruise ship.
As forts, they were cutting edge for the time: Divers had to place huge concrete rings down on sandbanks, about 20 or 30 feet below the water level, and stuff them with concrete slabs until the base peaked out over the water, according to the book "Ramparts of Empire."
Hundreds of soldiers were stationed at the sea forts. But the French never came after Portsmouth -- Napoleon III's reign ended in 1870, long before the Brits were even finished building -- and the pricey forts went down in history as "Palmerston's Follies."
Even though the view proved useless to the British military, it now works to the advantage of those looking for unique oceanside (or, rather, in-ocean) venues. Instead of keeping watch for the enemy, guests of the forts can luxuriate in ultra-private upscale lounges.
The forts' modern features include a dance club, a fine-dining restaurant, wine tastings, a fort-top hot tub, laser tag and game rooms. The company has big plans to host "Great Gatsby-themed parties, Bond nights and Royal Britannia-themed lunches," according to its website.
AmaZing Venues opened Spitbank Fort first, in 2012, then No Man's Fort in April 2015. The company plans to open the third fort, Horse Sands Fort, as a museum in 2016. Plans for the fourth remaining fort, St. Helens Fort, are not yet disclosed.