The Source of the Duero Nature · Duruelo de la Sierra, Pinares, Soria Image of Duruelo de la Sierra, not of the place itself. In this municipal district rises the river that gives its name to half of Castile: in its first twenty-six kilometres the Duero descends more than eight hundred metres from the slopes of Urbión, whose peak reaches 2,228 metres. Photo: Wamba Wambez (CC BY-SA 3.0) What to see and do in Duruelo de la Sierra