Uncastillo (Aragonese: Uncastiello) is a municipality in the province of Zaragoza, Aragon, eastern Spain. According to the 2010 census, the municipality has a population of 781 inhabitants. Along with the localities of Sos d'o Rei Catolico, Exeya d'os Caballers, Sádaba and Tauste it is one of the five towns that make up and give name to the comarca of Cinco Villas. In 1966 Uncastillo was declared a Conjunto Histórico-Artístico. It has several Romanesque buildings, including the Parish church of St. Martin, the parish church of Santa Maria, the churches of San Felices, San Andrés and San Juan. Other sights include the Castle and the Palace of Peter IV (14th century)."