Guide to the Great siege tunnels built following the Spanish siege of Gibraltar in 1779
Guarding the tunnels
At the end of the Great Siege, in 1783, the defeated Commander of the French and Spanish troops, the Duc de Crillon, on being shown the fortifications, primarily the Siege Tunnels, that had led to the defeat of his troops, commented “These works are worthy of the Romans”. High praise indeed from a vanquished foe.
Digging by hand
In June 1779 Spanish forces laid siege to Gibraltar during the American War of Independence. An army faced Gibraltar across the narrow isthmus connecting the Rock to the mainland, and the Spanish fleet patrolled the waters of the Strait and Algeciras Bay. The Spanish spent two years building up their forces and constructing the ‘Lines’, a series of fortified trenches and gun emplacements across the isthmus, in preparation for an assault on the British. A planned assault for November 1781 was foiled by the British in a now famous action, ‘the sortie’ when the garrison forces sallied out through Landport Gate and destroyed many of the forward batteries.
The North Face
Despite constant bombardment from the Spanish positions the defenders held on. Supplies periodically arrived by sea, small boats from Minorca, Lisbon and Morocco avoided the Spanish blockade to bring supplies to the garrison, and the occasional fleet from Britain slipped past the patrolling Spanish ships. Most of the civilian population departed for England in the returning ships.
Following the defeat of Cornwallis at Yorktown in October 1781, the French, allied to the United States, had effectively thrown the British out of North America. This left the French free to concentrate on the war in Europe. The French considered their Spanish allies’ efforts to take Gibraltar a travesty.
Kohler's depressing gun
In early 1782 the Duc de Crillon took over command of the siege and French units began to augment and reinforce those of the Spanish. By September of that year, the attacking forces consisted of at least 33,000 soldiers, 30,000 sailors and marines, 114 land guns and mortars, 47 ships of the line, 10 floating batteries and 40 gunboats.
General Eliott, in command of the British and Hanoverian defenders, had 7,000 soldiers, 500 gunners, 96 guns and 12 gunboats at his disposal. Any idea that would give an advantage to the defenders was welcomed, and during the period of the siege, there were a number of innovative ideas put forward by the rank and file. Some were implemented, and the inventor rewarded. One of the more radical ideas was the brainchild of a Sergeant Major Henry Ince.
In the May of 1782, General Eliott had offered a reward to anybody who could come up with a plan to get a cannon up to a particular high point on the Rock called 'The Notch'. Eliott wanted to give his gunners the chance to fire down into the enemy’s lines at maximum range. Sergeant Major Henry Ince of the Company of Military Artificers (later Royal Engineers) had the idea of mining a tunnel through the rock rather than trying to haul guns up the outside. They used crowbars, sledgehammers and gunpowder to drive a 2.4 square metre tunnel 25 metres into the limestone of Gibraltar in the first five weeks.
Small holes were blasted through the sides of the tunnel to ventilate the fumes from the gunpowder. It quickly became evident that these ventilation holes made excellent gun positions as they overlooked the enemy positions and offered good protection to the gunners. This first gallery was called Windsor Gallery, and by the time the siege ended, it was 113 metres long and had four guns in embrasures along its length. Two more tunnels were excavated before 1783 lower down the north face of the Rock called 'Kings and Queens Lines'.
The Artificers did not manage to reach 'The Notch' during the siege itself. Instead of installing a gun position above it they excavated a broad firing base within the rock and called it St. George's Hall.
Sergeant Major Ince received his reward, a horse from the Duke of Kent, Gibraltar's Royal Governor and father of Queen Victoria, and a parcel of land on the Rock. It is still called Ince's Farm. Tunnelling continued after the siege up to the end of the Second World War.
By the end of the 18th century, a series of galleries and interconnecting tunnels almost 1,200 metres in length had been created in The Rock.
Gunners high on the Rock had the advantage of height and therefore range, but they could not depress the guns far enough to shoot at attackers close into the rock. In 1782, Lieutenant George Frederick Koehler designed a gun carriage to solve this particular problem. The depressing gun carriage allowed this practically and innovatively. The gun itself was fixed to a sliding carriage which allowed the barrel to recoil upwards whilst the carriage itself remained fixed. Koehler’s second innovation was to attach the sliding bed to the carriage with a vertical spindle. This configuration allowed the sliding bed to rotate to the side which allowed the gunners to reload without being exposed to enemy fire. One of Koehler’s guns has been restored and is now mounted in Casemate’s Square.
The gun was tested in 1782. The chosen target was the San Carlos Battery, a Spanish gun position in their lines at a range of about 1,300 metres. 28 out of 30 shots hit the target which was pretty impressive at the back end of the 19th century.
A tour of the Great Siege Tunnels starts just above the Moorish castle where the entrance is now guarded by a cannon dating back to Victorian times.