Llantrisant ("Parish of the Three Saints") is a town in the county borough (principal area) of Rhondda Cynon Taf, Wales, within the historic county boundaries of Glamorgan, Wales, lying on the River Ely and the Afon Clun. The three saints of the city's name are Saints Illtyd, Gwynno, and Dyfodwg. Llantrisant is a hilltop settlement, at an altitude of 174 m (565 ft) above sea level. The town is home to the Royal Mint. It had a population of 15,313 in the UK census of 2011.
For more information, see the EN Wikipedia article Llantrisant.
The first three maps are provided by A Vision of Britain through Time
These maps were found on Wikimedia Commons
These maps of Glamorgan post-1974 were found on another site and are very useful for sorting out the up-to-date geography of the area