Llywelyn the Last | Gruffydd ap Llywelyn | Llywelyn the Great | Llywelyn | Morgan Llywelyn | Madog ap Llywelyn | Llywelyn the Elder ap Maredudd ap Cynan ab Owain Gwynedd | Llywelyn Siôn | Llywelyn Bren | Llywelyn ap Dafydd | Gruffudd Hiraethog | Gruffudd Fychan II | Gruffudd ap Cynan | Gruffudd ab Owain Glyndŵr |
Castell Du, also known as Sennybridge Castle or Castell Rhyd-y-Briw, is located approximately eight miles west of Brecon in Powys, Wales, and is believed to be the work of Llywelyn ap Gruffudd, Prince of Wales.
Madog was the son of Llywelyn ap Maredudd, the last vassal Lord of Meirionydd who had been deprived of his patrimony in 1256 for opposing Llywelyn ap Gruffudd the Prince of Wales at the Battle of Bryn Derwin.
However, problems with resupply in the event of siege and its destruction by Llywelyn ap Gruffudd, Prince of Wales in 1263 to prevent it falling into King Edward's hands, led to a new castle being built across the water in Conwy town.
The etymology of the name is unclear, with the personal name "Helen" or "Eleanor" being one possibility, perhaps after Eleanor de Montfort (d. 1282), princess of Wales and wife of Llywelyn ap Gruffudd.