'Martello tower' definitions:

Definition of 'martello tower'

noun

A circular masonry fort for coastal defence

From: GCIDE

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. [1913 Webster]
Note: The English borrowed the name of the tower from Corsica in 1794. [1913 Webster]