Martello Tower paintings by artist Cathal O'Briain. Martello towers, sometimes known simply as Martellos, are small defensive forts that were built across the British Empire during the 19th century, from the time of the French Revolutionary Wars onwards. Most were coastal forts. They stand up to 40 feet high and typically had a garrison of one officer and 15–25 men. Cathal enjoys sketching outdoors and working from photographs at his Dublin based studio and gallery.