Martello towers (or simply Martellos) are small defensive forts built in several countries of the British Empire during the 19th century, from the time of the Napoleonic Wars onwards.
They stand up to 40 feet (12m) high (with two floors) and typically had a garrison of one officer and 15-25 men. Their cylindrical structure and thick walls of solid masonry made them 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. A few towers had moats for extra defence. The Martello towers were used during the first half of the 19th century, but became obsolete with the introduction of powerful rifled artillery. Many have survived to the present day, often preserved as historic monuments.
There is some difference of opinion on the origin of this term, but one may accept that it is a corruption of Mortella and relates to the British naval attack on Cape Mortella in Corsica in 1794 where the defenders of a tower of this type were able to offer prolonged resistance to a superior force. As their success was ascribed to the design of the tower, similar towers were erected in England for coastal defence and they were also considered suitable for colonial conditions. Several were built in South Africa.
[Extracted from Wikipedia and Oberholster, J. 1972. The historical monuments of South Africa. Cape Town: Rembrandt van Rijn. p, 156.]