The A5 road makes a long steady climb between Bethesda, Gwynedd, and Llyn Ogwen in Conwy.

Lord Penrhyn's road, largely followed the valley floor, and at Capel Curig in 1801 he built a coaching inn, which is now Plas y Brenin, the UK National Mountaineering Centre.

Telford carved much of his road out of the north-eastern slopes of the Nant Ffrancon, thereby encountering difficulties in construction and future maintenance.

But this enabled him to observe a maximum grade of 1 in 14 along the whole route from London to Holyhead in order to facilitate horse drawn mail coaches throughout.

It has been used as a filming location for British film-makers, including Nant Ffrancon Golf Club (now defunct) appeared in the late 1920s/30s.