mar·tel·lo tower /mɑrˈtɛlo-/
Mar·tel·lo tow·er Fort. A building of masonry, generally circular, usually erected on the seacoast, with a gun on the summit mounted on a traversing platform, so as to be fired in any direction.
Note: ☞ The English borrowed the name of the tower from Corsica in 1794.
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martello tower
n : a circular masonry fort for coastal defence