|Length:||17.3 miles (27.8 km)|
|Meets:||A470, B4391, B4501, A494|
|Former Number(s):||B4391, B4470|
|Route outline (key)|
The A4212 opened in 1964 as a replacement transport corridor for the railway line between Trawsfynydd and Bala: a link which was severed by the construction between 1960 and 1965 of the Llyn Celyn dam and reservoir by the City of Liverpool. The artificial lake created by the dam flooded a large part of the Treweryn valley above Bala, together with the community of Capel Celyn and much of the B4391 Ffestiniog to Bala road where it passed through the area. The new road was built largely at the expense of Liverpool Corporation, builders of the dam and owners of the reservoir, and was authorized by the Bala to Trawsfynydd Highways (Liverpool Corporation Contribution) Act, 1960.
The first section, from the A470 at Trawsfynydd to the junction with the B4391 by Bryn Maenllofri quarry at Fridd Bwlch Llestri, ascends Cwm Prysor: a valley which prior to the building of the A4212 had had no roads other than farm tracks, although it was served by four railway stations. A well laid-out single-carriageway road almost entirely devoid of side turnings, it largely parallels the route of the former railway, although generally at a lower level in the valley, before a short ramp at 1 in 10 for around 800 m brings it up to the watershed where for a brief stretch it runs directly on top of the old railway alignment around the northern shore of Llyn Treweryn.
This small, natural lake marks the head of the Treweryn valley and shortly beyond it the B4391 from Ffestiniog comes in from the left. Formerly continuing from here to Bala – by way of the doomed village of Capel Celyn – and still extending onwards from there for a further 30+ km towards Llanfyllin, the B4391 has now been replaced down to the Dee at Bala by the new 1964 road, taking over much of its alignment except, of course, for the drowned section.
A picnic area overlooking Llyn Celyn on the right marks the approximate spot at which the new road deviated from the old B4391 in order to stay above the 300-metre contour until past the dam. To achieve this, a pronounced "buckle" in the route takes the road first north-by-northwest along one side of the flooded Afon Celyn side valley, then – after a near 180° curve across Pont ar Gelyn – back southeastward and past the Memorial Chapel (built in 1971).
After following the northern shore of Llyn Celyn – with views of the reservoir now somewhat restricted by subsequent tree plantation – the A4212 rejoins the original alignment of the B4391 close to Ciltalgarth, the improved road descending from here to the A494 at Bala and passing through the village of Fron-goch, where the B4501 from Cerrigydrudion comes in from the left.
There is some evidence that the route may not have been accorded Class I status until 1968 or later, with some road atlases published in the later part of the 1960s showing the brand-new section (from Trawsfynydd up Cwm Prysor to Bryn Maenllofri) with the number B4470 and the remainder of the itinerary, including the diversion around the north of the newly created Llyn Celyn, still labelled as B4391. In addition, Department for Transport records include an entry from before construction of the Cwm Prysor route reading: "B4470 - Reserved for new Class 2 road (Trawsfynndd - Cwm Prysor - Bryn Maen.)".