Caerau Hillfort was the subject of a forgery in a book called 'Gwentian Brut' in The Myvyrian Archaiology of Wales published in 1901. The forgery was fabricated by Edward Williams Iolo Morgannwg) while he was one of the editors of Myvyrian Archaiology; it suggested that Caerau Hillfort was the site of the "Battle of Rhiwsaeson" in 873 CE.

A smaller fort, Lle’r Gaer, with a much eroded single bank survives in Cwm Llwyd near Castellau. The Romans established a major iron smelting operation at Caergwanaf south of the Ely river from the 1st or 2nd century. There was also a Roman marching camp at Penycoedcae.

To the north of Llantrisant is an example of pre-Christian life at Tarren Deisant, a rock face with a natural spring on the west of Nant Castellau. Recorded in 1696 as “Tarren y ddau saint where two persons are engraved in stone, lying neere Cystelle”.  Possibly Celtic pagan belief regarded this as sacred waters but with the coming of Christianity and the use of two saints as the title doesn’t mean it was a holy well of Wales.

There are no historical references to Llantrisant by that name prior to 1246 when a Margam Charter makes references to the castles of “Neth et Landtrissen”. By the time of the death of Earl Richard e Clare there is evidence of a town emerging under the wall of the castle. The practice was for the lord of the castle to lay out building plots which would be let to new townsmen. 

However, evidently a sophisticated Celtic community lived here. The discovery of an inscribed cross slab dated to the 7th century implies an ancient settlement and references to fortifying the castle on an existing wooden fortress or offering a Charter of 1346 based on “ancient liberties” makes it clear that the hilltop has been occupied for many centuries.

With thanks to J. Barry Davies