The A4215 is a hilly route on the northern edge of the Brecon Beacons.
The road starts in Defynnog, at a junction with the A4067, a little way south of that road's northern terminus on the A40. The A4215 starts off by heading upstream (southeastward) along the valley of the Afon Senni, which gives its name to the village of Sennybridge just to the north. The hills on either side rise pretty steeply and it isn't that long before our road bears to the left and starts to climb out of the valley.
There's a continuous – but not particularly steady – climb for a couple of miles. The road levels out and crosses surprisingly fertile-looking fields. We cross a small valley then curve to the left and head east. The road narrows at the approximate location of the road's summit (which isn't obvious) then the white line down the middle of the road comes back as we start to descend more obviously. The descent isn't as steep as the climb, however. There are good views to the right overlooking Glyn Tarell before we drop down into the valley itself (losing the central white line again) and the road ends at a sharply angled triangular junction on the A470 just to the south of the village of Libanus.
The route was originally unclassified. By the mid-1950s it had gained Class II status (as the B4559) but it was only in around 1970 that the road was further upgraded to become the A4215.