Castle of Era Castèra Monument · Bossòst, Val d'Aran, Lleida Image of Bossòst, not of the place itself. Remains of a fortification on the rock of Casteràs, north of Bossòst, between the Garona river and the streams of Casteràs and Pèira Roja; the cave of Era Castèra was used first as a dwelling and later as a fortified refuge, a Celtic oppidum with epigraphic finds and coins from the Roman period. After the Treaty of Corbeil (1258) and the incorporation of the valley into the Crown of Aragon (1265) the site was reinforced, probably with a tower over the cave; today only wall fragments survive on the surface. A Bien Cultural de Interés Nacional (Catalonia's top heritage-protection category). Photo: Pere López Brosa (CC BY-SA 4.0) What to see and do in Bossòst