The southern section of the Narberth Road & Maenclochog Railway's 19¼-mile line from Clynderwen to Letterston Junction entered service on 19th September 1876. It formed a loop off the GWR main line, providing an alternative single track route into Fishguard Harbour.
On the southern approach to Maenclochog, the trackbed clings to the side of a steep valley, entering Castle Tunnel on a westerly curve. Masonry portals and short lined sections at either end give way to exposed rock in the tunnel's central part, incorporating refuges and ledges at a height of six feet on both sides.
On 6th October 1943, the tunnel played a small but notable part in the war effort, acting as the target for a prototype Highball bouncing bomb which was dropped from a Mosquito flying out of RAF Angle. By then, the passenger service had already been consigned to history but goods trains continued to ply their trade until 16th May 1949.
The old trackbed is now used a forestry access road. Vegetation largely obscures the south portal and there has been a small loss of brickwork from the lined section at that end.