The stony páramo of the río Chico Nature · Castilfrío de la Sierra, Almarza, Soria Image of Castilfrío de la Sierra, not of the place itself. At 1,204 metres of altitude, the municipal district is of stony ground and gives way to the river Chico, which runs down towards the Duero basin; 407 hectares are included in the Oncala-Valtajeros site. Photo: Diego Delso (CC BY-SA 4.0) What to see and do in Castilfrío de la Sierra