Sea forts are fortified stone structures located in the ocean. They can be found in the shallows near the coast, or set further off the mainland in deeper water. Built mainly to defend the approaches to a city or estuary, most sea forts were constructed on natural or artificial islands and would have provided a formidable obstacle to an invading navy. Today, many of these unique sea defenses still stand. But where can you admire some truly impressive and historic examples?
Click through and find out where waves meet walls.