Source : Webster's Revised Unabridged Dictionary (1913)

Martello tower \Mar*tel"lo tow`er\ [It. martello hammer. The
   name was orig. given to towers erected on the coasts of
   Sicily and Sardinia for protection against the pirates in the
   time of Charles the Fifth, which prob. orig. contained an
   alarm bell to be struck with a hammer. See {Martel}.] (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.

Source : WordNet®

martello tower
     n : a circular masonry fort for coastal defence