Round towers about forty feet in height, of great strength, and situated on a beach or river; so called from the Italian towers built as a protection against pirates. As the warning was given by striking a bell with a martello, or hammer, the towers were called Torri da Martello.
Some say that these towers were so called from a tower at the entrance of St. Fiorenzo, in Corsica. Similar towers were common all along the Mediterranean coast as a defence against pirates. They were erected in the low parts of Sussex and Kent in consequence of the powerful defence made (February 8th, 1794) by Le Tellier at the tower of Mortella, with only thirty-eight men, against a simultaneous sea and land attack- the former led by Lord Hood, and the latter by Major-General Dundas.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894