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.