The chestnut grove of Casillas Nature · Casillas, Arenas de San Pedro, Ávila Image of Casillas, not of the place itself. Extensive chestnut and oak woods cover the slopes of the municipality, at the very head of the Tiétar and on the southern side of the foothills of Gredos. They still carry economic weight and are one of the best preserved woods of the valley. Photo: Rodelar (CC BY-SA 4.0) What to see and do in Casillas