Castle of Bossòst Monument · Bossòst, Val d'Aran, Lleida A castle located north of Bossòst, on the rock known as Casteràs, between the river Garona and the streams of Casteràs and Pèira Roja, a strategic enclave for the defence of the Aran, since it controlled the routes of entry into the Baix Aran. It was a royal stronghold and the head of the old terçó of Bossòst, also called “de Lairissa”. After the Treaty of Corbeil (1258), which turned the area into a border zone, and its incorporation into the Crown of Aragon (1265), the site was strengthened and a tower was possibly built over the rock shelter that already existed in the rock, previously used as a fortified refuge, perhaps a Celtic oppidum. In 1283 the Aran was invaded by Philip the Bold; the castle, on top of the rock, was no longer in use by the 17th century. Today only fragments of wall remain. Bien Cultural de Interés Nacional (Catalonia's highest level of heritage protection). Photo: Pere López Brosa (CC BY-SA 4.0) What to see and do in Bossòst