Cefnllys is an abandoned ghost village, formerly a medieval castle town and borough, near Llandrindod Wells in Radnorshire (now part of Powys), Wales. The settlement was founded during the 13th century by the Mortimer family in order to strengthen their hold on land in the vicinity of the Cefnllys Castle, with which the town was closely associated. The town was probably intended to become the primary settlement of the lordship and cantref of Maelienydd, but was unsuccessful and declined during the 14th century as a result of bubonic plague outbreaks, economic isolation and military insecurity. Despite this, Cefnllys retained its borough status until the 19th century.