The parish church of San Esteban is the architectural centrepiece of Sos del Rey Católico, built in a late Romanesque style over the eleventh and twelfth centuries with Gothic additions made as the town grew in importance. The building rises above the town's rooflines and its tower is visible from the surrounding countryside.
Inside, the church is dark and cool, with well-preserved Romanesque capitals in the cloister and a notable painted altarpiece. The crypt — the Sala de la Reina — is where Ferdinand of Aragon was born and is now a small museum telling the story of the Catholic Monarchs and their significance for Spanish history. Entry to the crypt is included in the church ticket.