Cefnllys was a medieval town in Radnorshire (now part of Powys) in central Wales. Only St Michael's Church remains standing, with mounds indicating the remains of other buildings existing before the decline of the town in the 19th century. Close by on the hill known as Castle Rock are the remains of what is known as Cefnllys Castle. Almost encircling the hill, church and remains of the town is the River Ithon which continues to Llandrindod Wells a little over a mile away.