The Co-Cathedral of San Pedro is one of Soria's most rewarding church visits, partly because it is not just a single style frozen in place. The Gothic body of the church sits beside a Romanesque cloister, and the contrast gives the building more texture than its plain exterior first suggests.
It is close enough to combine with San Juan de Duero and the riverside walk, making it a good stop for travelers interested in Soria's medieval side. Opening hours can be limited, so check before building a day around the interior.