The tunnel was built in 1870 by the London & North Western Railway to carry a double-tracked railway southwards from Caernarfon Railway Station (nprn 41464) underneath Castle Square to join with the company's Carnarvonshire Branch and Carnarvonshire & Llanberis Branch. The lines through the tunnel closed in 1964 and after several years of lying disused, the tunnel was converted to carry road traffic in about 2000.
B.A.Malaws, RCAHMW, 13 February 2014.