The B4578 follows part of the Roman Sarn Helen in central Cardiganshire. It is a shorter but hillier alternative to the A485, which it meets at both ends. The road was originally unclassified but gained its number in 1935.
Traffic heading south along the A485 from Bronant reaches Tyncelyn, where the road bends sharply left; the road continuing ahead is the B4578. The B4577 turns off to the right immediately afterwards then we continue south along a narrow road, typical for the area. We cross a small valley, with steep gradients on either side of the stream, then continue across relatively level fields. We pass a turn to the isolated Llanbadarn Odwyn Church then continue to Stags Head, where we cross the B4342.
We continue south along the dead-straight Roman road across gently undulating terrain. It's not long before we reach the end of the road at a T-junction back on the A485 at Llanio.