Reconstruction of buildings, Los Millares prehistoric Chalcolithic settlement archaelogical site, Almeria, Andalusia, Spain, Europe

Martello Tower, with walls up to 11 feet thick and surrounded by dry moat, at Ferry Reach, built by the British Army in 1883 to protect the main channel into Bermuda, St. George's Island, Bermuda, Atlantic, North America

Martello Tower, with walls up to 11 feet thick and surrounded by dry moat, at Ferry Reach, built by the British Army in 1883 to protect the main channel into Bermuda, St. George's Island, Bermuda, Atlantic, North America

Detail from the Citadelle Fort, built in 1817 by Henri Christophe, the walls are four metres thick, Milot, Haiti, West Indies, Caribbean, Central America

Approach to the Citadelle Fort, built in 1817 by Henri Christophe, the walls are four metres thick, Milot, Haiti, West Indies, Caribbean, Central America

Side view of a castle fortified by thick walls, san Juan, Puerto Rico