from Annanova - Built between 1861 and 1880,  it took a huge engineering effort to cut vast blocks of granite and transport them by barge to the building site before they were lowered onto the sea bed for the fort's foundations.

Nearly 200 feet across and rising 60 feet from the sea, its   water supply comes from a borehole sunk into the seabed. It also has electricity generators.

In recent times,  the fort has served as a luxury hotel complete with 21 sumptuous-themed rooms, two helipads and heated indoor swimming pool. But the company has collapsed and the fort is on the market.

Rachel Halliday, spokeswoman for administrators KPMG, said: "We are letting people express an interest in the property before we invite offers for it, but it's expected to fetch up to £4 million

"This property is certainly a one-off and we have never had to sell anything this unusual before."