The British built 15 Guernsey loophole towers at various points along the coast of Guernsey between August 1778 and March 1779 to deter possible French attacks after France had declared itself an ally of the Americans in the American Revolutionary War. Towards the start of the Napoleonic Wars several towers received additional reinforcement in the form of batteries at their bases. Today, 12 towers still survive, three having been destroyed at different times. Two of the survivors, Petit Bôt and Rousse, contain interpretive exhibits that the public may examine.
In 1778, General Henry Seymour Conway, Governor of Jersey, recommended that 30 towers (see Jersey Round Tower) be built there to impede a possible French incursion. As it happened, almost all the towers were built after the Battle of Jersey in 1781. However, in July 1778, the British government also authorized the building of 15 towers on Guernsey.[1] These towers were designed to accommodate only muskets (i.e., there was no provision for artillery); though there was discussion of giving each tower a Coehorn mortar for the roof platform nothing came of this.[2]
Although most of the towers were built on the Commons, or on public land above the high-water mark, three towers were to be built on private land. The States were of the opinion that the project was of such importance that if necessary they would exercise eminent domain, "...notwithstanding any Clameur de haro or any opposition whatsoever...".[1]: 19–21
Manning the towers was the responsibility of the Royal Guernsey Militia. This force of 4-5,000 men consisted of a field artillery regiment and four infantry regiments. Service in the militia was compulsory and unpaid. The men took turns manning the towers and other fortifications assigned to each regiment.
A report of 1787 pointed out the towers' limitations.[1] As a result, at the onset of the Napoleonic Wars, and during General Sir John Doyle's tenure as Lieutenant Governor (1803-1816), a number of the towers received supporting batteries, either at their base, or nearby. Also, in 1803, the towers had their roofs strengthened and they received 12-pounder carronades as additional armament.[2]
During the German occupation of the Channel Islands during World War II, the Germans modified some towers to their purposes, generally by replacing some of the loopholes with machine-gun slits. The Petit Bôt Tower provided one example; however, its recent restoration returned it to its pre-World War II form.
The towers were all built to one design. They all had three floors, the two above the ground floor having loopholes that would permit musket fire to cover all approaches to the tower. Captain Frederick Bassett, RE, the Commanding Engineer in the Channel Islands, oversaw the towers' construction.[2] They were to be built to the design of the Jersey round towers, but in fact deviated in a number of ways. They had a batter (slope) at the base, and were slenderer than the Jersey towers. The Jersey towers also later received machicolations, which are absent from the Guernsey loophole towers.[1]
The towers are 30 ft (9 m) tall, and have an external diameter of 20 ft (6 m) and an internal diameter of just over 12 ft (3.7 m). They have a batter to the walls to the first floor, and then rise straight up from there.[1]
The towers were built of Guernsey granites. Tower #1, at Houge à la Perre Battery, on Belle Greve Bay, St Peter Port, was built of St Peter Port gabbro. All the others were built of local granites, with Vazon being built of Cobo Granite (Adamellite).
The towers are numbered sequentially, in a counter-clockwise direction from St Peter Port.[1]
