If you put Cynefin: S4C in the search engine on YouTube, there are very interesting explanations of many names such as London, Llandudno and Casgwent (or Chepstow).
Os rhowch Cynefin: S4C yn y chwiliwr ar YouTube, mae yna esboniadau diddorol iawn gan Tudur o sawl enw fel Llundain, Llandudno a Chasgwent.