Martello towers are small defensive forts that were built across the British Empire during the 19th century, from the time of the French Revolutionary Wars onwards. They stand up to 12m high (with two floors) and typically had a garrison of one officer and 15–25 men. Their round 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, and hence fire over, a complete 360° circle.
My friend was in Antigua a number of times but it is only last year that he managed to also visit Barbuda - which is situated north of Antigua and has a population of about 1638. He managed to find one single view card - literally! how lucky for me :))