Prosperous medieval town dominated by the castle of the viscounts, the most powerful in Catalonia. The residents of Castellbò, always faithful to their lords, were the authors of the stoning of the inquisitor Pere de Cadireta, which is why they earned the nickname matasants, already collected by Francisco de Zamora at the end of the 18th century and which is still valid today.

Almost nothing remains of the castle, it was demolished in 1513 by order of King Ferdinand the Catholic. However, the collegiate church of Santa Maria stands at the foot of the castle hill, preserving all its monumentality.

A few kilometers east of the town, the ruins of the hospital priory of Costoja are hidden in the middle of the forest. These ancient stones are the mute witnesses of the terrible posthumous condemnation that the Inquisition threw against Viscounts Arnau and Ermessenda.

• Arnau de Castellbò:Son of a family of high Pyrenean aristocracy, Viscount Arnau de Castellbò inherited a serious dispute with the bishops of Urgell that would favor his approach to Catharism, to the point of promoting the establishment of a deacon in the town of Castellbò, around 1220. Arnau did not come to actively profess the faith of the Good Men, although he would die consoled in 1226.