Llangollen Bridge is thought to have been built by Henry I, it was probably reconstructed in 1346 by Dr. John Trevor (Bishop of St Asaph 1346-1357), and then replaced in the 16th century. The bridge was extended at the north end to span the railway line in May 1863, and the parapets were rebuilt. The width of the bridge was greatly widened on the upstream side in 1873 and in 1968 it was again nearly doubled in width to the same side. The river is spanned by 4 arches, unequally pointed and with voussoirs, of coursed rubble to the older part, precast concrete to the upstream side. There are deep pointed cutwaters to both sides (taller to downstream side) and a square-headed railway opening.
Source: DE/DOM/SJ24SW, from the Cadw listed buildings database
J. Archer, RCAHMW, 19.10.2004.