Martello towers, are small defensive forts that were built around the Irish Coast (and across the British Empire) during the 19th century, from the time of the French Revolutionary 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 round structure and thick walls of solid masonry made them resistant to cannon fire. Martello towers were inspired by a round fortress, part of a larger Genovese defence system, at Mortella (Myrtle) Point in Corsica, completed in 1565. Since the 15th century, the Corsicans had built similar towers at strategic points around the island to protect coastal villages and shipping from North African pirates.
Possibly the most famous Martello tower is the one in Sandycove, near Dublin, Ireland in which James Joyce lived for a few days. Joyce shared the tower with Oliver St. John Gogarty, then a medical student but later to become famous in Irish history as a surgeon, politician and writer. In Ulysses, the fictional character Stephen Dedalus lives in the tower with a medical student, Malachi "Buck" Mulligan, whom Joyce based on Gogarty. The James Joyce Tower, as the tower is now known, houses a museum dedicated to James Joyce. Reference: Wikipedia.