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.
The 'castle' was actually two, built one after the other at opposite ends of the hill by the English lord Roger Mortimer of Wigmore during his battles with Llywelyn ap Gruffudd, Prince of Wales, for the cantref of Maelienydd. The first, at the north end, was built in 1242 and ruined in 1262 by Llywelyn. The second, at the south end, was built in 1268 and survived Llywelyn's death, but was sacked by Cynan ap Maredudd during the revolt of Madog ap Llywelyn in 1294-5. It was ruined by 1588. 
- Cefnllys Castle
- History & Architecture of Church of St Michael, Cefnllys from Radnorshire Churches Survey Project of the Clwyd-Powys Archaeological Trust
- "Romance in an old church" from powys.org.uk
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