Prince’s Line Engine Room north chamber.

Ref: HLCGFP4/018

After the capture of Gibraltar in 1704, the British exploited the natural platforms flanking the northern approaches to the Rock using much of the geographical landscape to their military advantage.

The Board of Ordnance writing to the Chief Engineer Jonas Moore on the 2^nd January 1727, mentions the use of caves for defensive purposes:

'The Last of December Me sent you Orders to Erect, without loss of Time, Cover'd Batteries Bomb proof in the Kings and Princes Lines retaining walls to prevent the enemy's Bombs rowling down into those Lines, and such other works as you shall find necessary for the better Defence of that place. As also to put Mines in the Glacis of the North Gate into the best condition they are capable of, they being what was the greatest Terrour to the enemy during the last Siege. All of which we desire you will punctually Execute with the utmost Exactness, Frugality and Dispatch. And the better to inable you thereto, altho Me have yet no money allow'd us for the Works at Gibraltar, Me shall answer any bills the Storekeeper by your direction shall draw upon Us, not exceeding Two Thousand Pounds Sterling…. There are two large Caves, one in Princes Line and the other in the Kings Lines, Me wou'd have you to consider if those Caves can be of any use to cover the Men (in case of a Siege) from Bombs or Stones, that if you find them capable of such use you may make them Commodious for the Service, either by enlarging and making Benches or other Conveniencys as you shall find most proper. Besides a Large Supply of Stores which Me hope will arrive with this Letter or very soon after it. Me send over two Engineers to be Assisting you and a Reinforcement of 30 Gunners etc.'

Caves and natural rock faults were therefore used 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.

By 1790 over 4,000 feet (1,200 m) of tunnels had been excavated, providing bombproof communications routes between the various lines and batteries on the North Front of the Rock.

Northern Defences tunnels and caves complex.

The Prince’s Line North Chamber lies almost directly above the Star Chamber. This cave was discovered in 1790 when they built a gallery linking Castle Communication and Hanover Battery with the Prince’s Lines. Within this Gallery a ramp named Lower Prince’s Gallery, leads down to the Star Chamber which was a major access route for guns to the lower lines. The whole system of galleries was finished by 1800 and no other large-scale tunnelling programme was to be carried out until 1940. The tunnels and galleries within the Northern Defences were therefore the first underground chambers to be used by the garrison during WWII.

The Prince’s Lines engine room, however, was of a much earlier design dated to 1900 as a subsidiary power station operating independently of the main King’s Lines electricity station which had been built ten years earlier. The sub-station was completed in April 1904. This engine room therefore preceded all other underground military generating stations (AROW, Gort’s, Calpe Hole and Fordham’s) by over forty years. The Prince’s Line generators supplied electricity exclusively to the tunnels and defensive positions along the Northern Defences, which would later include the two strategic DEL (Defence Electric Light) positions covering the frontier and airfield from Prince’s Lines with the Directing Station located at Upper All’s Well. In November 1911 these lights were numbered 16 and 17. A third light was added in 1917. In 1940, these lights were reorganised and became F3, F4 and F5 of a system of Frontier Lights. Their primary role was for the illumination of the North Front to aid operations against an attacker but they could cover the water to the north of the Commercial Mole and to the north east of the Rock if required.

The engine room itself consisted of a main chamber and another for oil storage; between these was a small space for a workshop. Power was supplied by two 25HP Hornsby Akroyed oil engines linked to dynamos. The oil storage tanks had a capacity of 600 gallons. There was a further oil reserve storage within the lower galleries The second, western chamber was not part of the original design and was probably cut at a later date. Rollo states that in May 1941, a 75 mm gun was sited at the parapet of Princes Lines just outside the DEL engine room. It was kept in the nearby tunnel and run out when required in an anti-landing craft role.

There were in addition, three ventilation/exhaust shafts within the system, two of them within the main engine room with visible cubic structures above, the third was located in the furthest chamber and is currently topped by a manhole opening located within the trench above.

The system had two adits at the northern end, one directly under the entrenchment wall, the other some metres along the Prince’s Line from the first; both are currently bricked up.

The corrugated iron roof is original to the 1900 design.