It is clear that the British were using caves and natural rock faults as part of the Northern Defences from a very early stage, but it was not until after the Great Siege building on the success of the Great Siege Galleries that the soldier artificers turned their attention to excavate new galleries and tunnels in this area. Most of these works were carried out between 1787-1790.
One of the areas excavated and which revealed a number of natural caves was the rocky outcrop around a natural fault named the Orillon which divided the 18th Century King’s and Queen’s Lines. The base of the natural fault was covered by the Inundation, an artificial lake created to obstruct landward access to Gibraltar. The Orillon batteries were part of a three-storey gun position within a natural fault in the Rock.
- G. M. Goodwin gives the best account of this complex:
'This gallery leads into a fairly large chamber which, according to the 1819 report, was a second Orillon Battery. Into the walls of this emplacement, a few feet up, have been cut two wide ledges. These supported large wooden planks, which formed a platform for a gun and was called the Third Orillon Battery. A fourth Orillon Battery existed on the roof above the third, and a wooden staircase, in the recess of the rock behind, established communication between the second, third and fourth batteries. From what was the third Orillon Battery, a small gallery in the rock behind communicates with Queen's Gallery East, near the loop holed wall. In later years, the second Orillion was used as a bake house but the ovens have now been removed. Steps from the second Orillon lead down to a cave where three guns were mounted, forming the first Orillon Battery. At the back of the cave is a small shelter or magazine'.
The Queen’s Gallery, cut in 1789, was further divided into three parts; Queen's Gallery East, Queen's Gallery South West and Queen's Lines Gallery. A natural cave was found in Queen’s Gallery East next to Forbe’s Shaft.
The Star Chamber was formerly a natural cave, extended in 1790. It is reached from Prince's Lines by an overhead staircase but the bottom steps are now 70 feet above the floor level. This cave was literally and organisationally at the centre of the tunnelling operations of the late 18th century by the Soldier Artificer Company (later the Royal Engineers). During World War II the tunnels were still being used and brick buildings were constructed within the cave and galleries. A sign on the wall of the cave records that in 1941 this cave was the King's Regiment Battalion Headquarters.
Immediately to the North of the Star Chamber is another smaller cave formation called St. Patricks Chamber which in turn leads to a number of other natural caves which were named as Common’s Hall, Raleigh Gallery and 2nd and 3rd Orillon Batteries.
North of St. Patrick’s Chamber further tunnelling revealed a deep fissure which was found to part of the natural fault line leading to the Orillon. Water along this fault line exited at the Orillon as a natural spring. This natural cave was named Smart’s Well Reserve.