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.

a circular masonry fort for coastal defence

a gun tower designed solely as a defence against ships armed with guns

small circular or oval stone tower, often used in coastal defenses. Construction of three towers was commenced near Trincomalee c.1817.

Circular coastal tower once used for defence.

Martello towers (or simply Martellos) are small defensive forts built by the British Empire during the 19th century, from the time of the Napoleonic Wars onwards. They stand about 40 feet (12m) high (with two floors) and had a garrison of one officer and 25 men. Their round structure and thick walls of solid masonry made them very resistant to cannon fire, while their height made them an ideal platform for a single heavy artillery piece, mounted on the flat roof and able to traverse a 360° arc.