During the American War of Independence, France and Spain launched a major campaign to recapture Gibraltar from Britain. This became the 14th and most famous siege of the Rock — The Great Siege — which lasted from July 1779 to February 1783.

By 1782, despite continuous British counter-fire, Spanish forces had slowly advanced their trenches across the isthmus, inching closer to the British lines. As they approached, British artillery became less effective: the steep cliffs of Gibraltar’s North Front left little room for guns, and by May 1782 the Spanish had disabled many British batteries without facing significant return fire.

Faced with this growing threat, General George Augustus Eliott offered a reward of 1,000 Spanish dollars for a solution that would allow flanking fire against the Spanish works. Sergeant-Major Henry Ince of the Artificer Company proposed an innovative plan — to tunnel through the Rock itself. The gallery would begin above Farringdon’s Battery and emerge at a rocky outcrop known as the Notch, where a cannon could dominate the entire North Front. Eliott approved the idea immediately, and on 22 May 1782, Colonel Green issued the order:

“A gallery six feet high and six feet wide, through the rock leading towards the Notch nearly under the Royal Battery, to connect with a proposed battery at that point, is to be undertaken immediately by 12 miners under the direction of Sergeant-Major Ince.”

Great Siege tunnelers at work.

Excavation began on 25 May 1782 and was done entirely by hand. The miners used various techniques to break the limestone, including gunpowder blasting, fire-setting (heating the rock with fire and cooling it rapidly with water), quicklime expansion, and wooden wedges soaked with water to split the rock. Debris was cleared with crowbars and sledgehammers. Progress was slow — only about 200 metres (660 ft) per year — but the resulting tunnels proved remarkably stable and remain accessible today.

Initially, poor ventilation hampered the work. To solve this, the miners blasted a small opening in the cliff face, which not only provided fresh air but also revealed a perfect firing position. Colonel John Drinkwater Bethune, who wrote an account of the siege in 1785, described how this came about:¹

“The mine was loaded with an unusual quantity of powder, and the explosion was so amazingly loud, that almost the whole of the Enemy's camp turned out at the report: but what must their surprise be, when they observed where the smoke issued! – The original intention of this opening, was to communicate air to the workmen, who before were almost suffocated with the smoke which remained after blowing the different mines; but, on examining the aperture more closely, an idea was conceived of mounting a gun to bear on all the Enemy's batteries, excepting Fort Barbara: accordingly orders were given to enlarge the inner part for the recoil; and when finished, a twenty-four-pounder was mounted.”

By the end of the siege, the newly created Upper Gallery housed four guns mounted on specially designed depressing carriages, enabling them to fire down into Spanish positions.

Gun embrasure within the Great Siege Tunnels.

Although the tunnel never reached the Notch before the siege ended, the outcrop was later hollowed out to form a broad gun emplacement — a lasting testament to the ingenuity and determination of Gibraltar’s defenders.

By the end of 1783, the total length of the tunnels excavated was approximately 908 feet (277 m). When the diggers eventually reached the Notch, plans changed: rather than placing a single gun on top of the outcrop as originally intended, they hollowed it into a large chamber known as St. George’s Hall, which was later fitted with seven guns.

In total, five galleries were excavated: Windsor Gallery, King’s and Queen’s Lines, St. George’s Hall, and Cornwallis Chamber. The Windsor Gallery was the first section to be completed and housed four guns. St. George’s Hall was the largest of the original galleries, and its embrasures are still visible today on the slopes of the Rock when approaching Gibraltar by land or sea.

Members of the Soldier Artificer Company in working dress 1786.

Inside a gallery on the Rock of Gibraltar, engraved by I.C. Stadler after Rev C. Willyams (c. 1800), depicting one of the tunnels excavated by Henry Ince.

In 1878, the batteries in the Upper Galleries were equipped with 64-pounder RML guns — one facing west and three facing east. These remained in place until 1908, when the last 64-pounder RMLs were removed from the approved armament. One of these guns is still on display outside the entrance to the Upper Galleries.

See Link:  64-pdr 58 cwt RML, Upper Galleries.

St. George's Hall with 64-pounder 58 cwt RML guns on display.

¹ John Drinkwater Bethune, Colonel: A history of the late siege of Gibraltar.: With a description and account of that garrison, from the earliest periods. (1786)., p. 247-248.