This is a great route which links the Tremadog area to Ffestiniog via the B4410, without cheating and going on the flatter main road. It is a bit brutal, but rises in a number of steps so you do get a breather at fairly regular intervals. The reward for all that effort is a nice descent eastwards as you approach the main road.
If done westwards it’s a bit grim and very steep in two separate sections.
The highest point of the road is only about 150m; consequently it would be a rare event for this road to be blocked with snow during the winter.
The route is often done as part of a longer ride, so there are numerous potential starting points; Tremadog or Beddgelert are obvious options.