When writing in pre-modern eras, one of the hardest things to discover, even with the internet, is the traveling routes across the countryside of those earlier peoples. In Wales, the Romans built roads but also improved old ones, which wasn’t their normal operating procedure. It was forced upon them, however, because they found the land so inhospitable that it made it difficult for them to lay down their straight roads.
In Gwynedd, in particular, the Romans built a road from Chester to Caernarfon. Instead of following the coastal plain, as the modern highway now does, it skirted the rock formations along the coast, running through St. Asaphs, curving north to Caerhun where it crossed the Conwy, took the ancient Welsh track between the standing stones at the pass of Bwlch y Ddeufaen, and then back down to the coast at Aber.
This is the road that Welsh people used from ancient times, through the Middle Ages and beyond. The best maps in for this era in Wales are the Ordnance Survey maps of Roman Britain and Ancient Britain, and online: http://www.multimap.com/maps/
Make sure you choose the ‘ordnance survey’ map, to show all the castles, forts, roads, and ruins.