The Río Duero long marked the frontier between Christian and Arab territory. It meanders right across central Castile, between Zamora and Soria, with the eastern section in particular, from Valladolid, marked by a series of spectacular castles and old market towns, some restored as tourist attractions, others crumbling to dust. This part of the river is also at the heart of one of Spain’s greatest wine-producing areas, the Ribera del Duero. It’s a fine route to follow by car (N122), stopping off for lunch in rustic posadas and for walks in the beautiful surroundings. You can make the trip by bus, but if you do, realistically, you’ll only be able to see the major towns of Peñafiel and El Burgo de Osma.