Grain Sea Fort
You will need to know the tide times or you will become trapped.
The military building defended the nation during the First and Second World Wars, from the mouth of the River Thames in Kent.
The ex-army site is only accessible twice a day, at low tide via a causeway and owners and guests will have to use a boat at any other time.
It was the last gun tower of its kind to be built, constructed to protect the nearby military dockyards against French invasion. Anglo-French tensions ran high in the 1850s and the nation feared a naval attack.
The tower guarded the key link between the Thames and Medway rivers, which led to Royal Navy Dockyards in Sheerness and Chatham.
However, artillery technology quickly improved and the construction became obsolete in the mid-19th century, almost immediately after it was completed.
By the end of the century it was transformed into a defence against raids by fast torpedo boats.
Decades later it was altered again - new, quick firing guns were added during World War I and World War II. The property off the Isle of Grain, built in 1855 was decommissioned in 1956.
