It was during the war of American Independence, when France and Spain made an all out attempt to recapture the Rock from the British in Gibraltar's 14th Siege, recorded as the Great Siege, which lasted from July 1779 to February 1783.
The Governor, General Elliot (later called Baron Heathfield of Gibraltar) is said to have offered a reward to anyone who could tell him how to get guns onto a projection from the precipitous northern face of the Rock known as the 'Notch'.
Sergeant Major Ince, a member of the Company of Soldier Artificers, forerunners of the Royal Engineers, suggested that this could be done by tunnelling through the Rock. Permission was granted, and Ince started work under the direction of Lieutenant J Eveliegh, a Royal Engineer, Aide de Camp to the Governor, on May 25th, 1782.
The tunnellers relied on the strength of their arms, on their skills with a sledgehammer and a crowbar, and were also aided with gunpowder for blasting. In five weeks 18 men had driven a tunnel 8 square feet (2.4sq m) by 82 feet long (25m) into the Rock. It is interesting to compare this with the record of a fully mechanised tunnelling company in Gibraltar during World War II, who in a week advanced 180 feet (55m).
Originally there was no intention of mounting guns in this gallery, but as the work progressed the fumes from repeated blasting almost suffocated the miners, so it was decided to open a vent to let air into the tunnel. Almost at once it was realised what an excellent embrasure this would make for a gun, so one was mounted without waiting to reach the 'Notch'.
Other embrasures were cut and guns mounted, and by the time the Siege ended in February 1783, the tunnel was 370 feet (113m) long and had four guns mounted in it. This first gallery was called 'Windsor Gallery'. Sergeant Major Ince did not stop there - he went on to tunnel two other galleries called "King's And Queen's Lines" lower down the north face of the Rock.
Work did not stop with the end of the Siege, but instead of continuing straight towards the 'Notch', a tunnel was driven downwards and a large chamber opened under the 'Notch' called St George's Hall, where a battery of seven guns where installed. The Cornwallis Chamber was also excavated at this time.
In gratitude to Sergeant Major Ince, he was given a Commission and granted a plot of land on the Upper Rock still known as Ince's Farm. In addition, the Duke of Kent (Gibraltar's Royal Governor and father of Queen Victoria) presented him with a valuable horse.
The tunnels have been lit and different scenes set.
Here a guard calls out 'Halt Who Goes There' as you approach the door.
Here the tunnel is quite high. You can see the roughly cut surfaces on the wall. An amazing piece of engineering.
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 that had led to the defeat of his troops commented, "These works are worthy of the Romans". This comment highlights the ingenuity of those men who, against all odds endured the onslaught of the advancing forces and were still able to devise a unique system of defence which afforded them victory.
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The Pillars of Hercules was the phrase that was applied in Antiquity to the promontories that flank the entrance to the Strait of Gibraltar. 
