Bwlch y Ddeufaen is a pass along the ancient road from Caerhun to Aber. The topography of North Wales is such that no significant road could run along the coastline due to the cliffs that come right down to the Irish Sea.
Thus, from ancient times, the people of Wales used a road that crossed the Conwy River at Caerhun and headed into the hills, reaching a pass marked by two standing stones on either side of the road. The road then descended out of the hills, arriving at Aber and was then able to follow the road west towards Bangor and Caernarvon.
Even the Romans found the topography impossible and chose to improve the ancient British/Celtic road rather than build an entirely new one closer to the coast. 1000 years later, the Normans faced the same difficulties. It was what allowed the Welsh Princes to rule from Aber almost uncontested. Protected by the mountains behind, their llys or court was accessible only by the pass of Bwlch y Ddeufaen or from the sea.
This road plays a role in many of my books.
Just to note that Bwlch y Ddeufaen means pass of the two stones (from dau – two and maen – stone, the d mutates to dd and the m to f in Welsh). I think you have confused it with defaid which mutates to ddefaid (sheep). Also the cliffs of Penmaenmawr and Penmaenmawr are igneous (rhyolite) rocks not limestone.
I will fix it. Thanks!