A 3.5km section of 'Sarn Helen', the Roman road, crosses common land on the north-east side of Llan Ffestiniog, mainly in the valley of Afon Gamallt. This road linked the forts of Canovium and Tomen y Mur. The road continued in use in medieval and post-medieval periods, and was relegated to a mere footpath probably only in the later nineteenth century. During its long period of use it has been resurfaced and so no Roman road surface is visible, although there is an undisturbed section of the road 600m long SW-NE at Bryn-y-Castell, centred on SH7267143039, where the road is cut into a slight slope to the east, with upcast banks on the downhill (west) side, and is 3.5m wide in total. This section is preserved because the road was later redirected on the east side of Bryn-y-Castell fort, rather than the west. The road crosses Nant Gamallt at Rhyd Helen at SH7295943284, but no structures or setts are visible.
To the north of the ford the route takes the high ground between Gamallt and Teigl valleys. The course is not straight, as it navigates its way between rocky hillocks. Sections around SH7332443907, it is 3.5m wide, terraced and embanked, with a low revetment on the south-east side. Another embanked section is around SH7337244114, where there is a retaining wall on the west side 16m long. In this rocky section there are two small contemporary roadside quarries (nprn 536616, 536684).
Towards the north end of the common, around SH738644517, the road consists of a ledge 4m wide across open moorland, now covered in moss and grass.
Recorded as part of RCAHMW Uplands Initiative Project, W B Horton, H & H, 19/11/2013.