The "Martello" towers that line the Irish coast at intervals north and south of Dublin (12 to the north, 16 to the south) were built in the early years of the 19th century by the British government, then under the leadership of "Billy Pitt" (Prime Minister William Pitt the Younger), to protect against the threat of invasion "when the French were on the sea."
The Napoleonic Wars between Britain and France, which had been brewing ever since the French Revolution, began in earnest in 1803. Fearing that French forces would invade to assist the cause of Irish revolution, the British constructed a series of small defensive fortifications along the coasts of England and Ireland, beginning in 1803 and ending with Pitt’s death in 1806. The tower at Sandycove, by some accounts, was the first in this string—built to protect the approaches to Dublin (personal communication from Robert Seidman). If true, this primacy may partly explain Mulligan’s saying that “ours is the omphalos” (i.e., the navel or source of all the others).
The British took as their model for all these fortifications a squat round tower constructed by the Genovese at Punta Mortella (Myrtle Point) in Corsica, which two of their own warships had tried to destroy in 1794. Their combined firepower of 104 guns failed to bring down the tower. Most defensive fortifications enjoy the advantage of height: they can fire down on attacking ships, while the ships’ guns must use a good portion of their power simply to overcome gravity. In addition, the design of this tower was brilliant. Its roundness, combined with a gentle slope inward from base to top, deflected much of the power of incoming shot, and whatever force was not deflected was absorbed by exceptionally thick walls. Additionally, its round artillery platform meant that it could direct fire around an unlimited arc. After two and a half hours of close bombardment HMS Fortitude was herself badly damaged by shot from the tower’s two eighteen-pounders, and the warships were forced to withdraw. The tower was eventually captured by British land forces after two days of fierce fighting, but the British were impressed and resolved to copy the highly effective design.
After ringing England and Ireland with similar towers (called Martellos, in a garbled recollection of Mortella), they went on to build many more throughout the Empire during the first two thirds of the century—about 140 in all. The towers were finally made obsolete in the 1860s and 1870s by the introduction of more accurate and powerful rifled artillery on warships.