It is definitely a hike from Blaenau Ffestiniog (200m altitude) to the quarries around Maenofferen (400m altitude but can be avoided. Take the small road from Llan Ffestiniog north east up the Afon Teigl valley for around 3 miles to the small car park outside the Manod quarry (what3words modem.singled.surpasses). That's 500m altitude reached without breaking into a sweat. A few yards north join the Rhiwbach tramway for the 3km walk on the level, passing Llyn Bowydd, see photo, which brings you out above the Maenofferen complex. Still an incline but not a difficult one.
Coflein is the online database for the National Monuments Record of Wales and has some good mapping as well as descriptions of thousands of Welsh archaeological sites:
