The 19th-century author Louis Fédié, who was also an amateur historian and the president of the Société des Arts et Sciences in Carcassonne, popularised the claim that the village of Rennes-le-Château was the ancient city of Rhedae in his 1880 book Le Comté de Razès et le diocèse d'Alet.

Louis Fédié’s identification was and still is widely disputed by professional archaeologists and historians and the precise location of Rhedae remains unknown.

Rhedae was probably located near modern Quillan – a major crossroads town about 20 kilometres south of Limoux in the heart of the ancient County of Razès, now in the Aude. Limoux was the former capital of the Razès.

Bill Putnam and John Edwin Wood in their book The Treasure of Rennes-le-Château: A Mystery Solved (2005) noted the difficulty in understanding how the name Rhedae could have been changed to Rennes-le-Château. They noted that Rhedae was cited as being “located at the crossing point of four major roads: how could this possibly be the case at Rennes-le-Château on its isolated hilltop?”

Below, an ancient coin minted in the County of Razès bearing the letters R. E. D and S

And from Dom Jean Joseph Vaissète, Géographie historique, ecclésiastique et civile, Ou Description de toutes les Parties du Globe Terrestre, enrichie de Cartes Géographiques, Volume 3, 1755:

“The other most important places in the part of the Diocese of Alet which is dependent on Le Rasez are: Quillan, a small town of 400 households on the Aude, situated about 6 miles below and to the south of Alet. The Archbishop of Narbonne is its lord. As a diocesan town it is entitled to send its First Consul to the provincial parliament every six years. Arques, a barony, whose baron has the right to sit in the parliament of the Languedoc. Esperaza and Couissan, two large villages, are dependent upon Arques. Rennes, a place famous for its thermal baths. Archaeological excavations there have uncovered ancient medals and inscriptions, which tells us that it was known in Roman times. It seems that Rennes is located on the same site as the ancient castle of Redae, after which Le Rasez is named.”