The parish church is dedicated to St. Stephen protomartyr stands in the immediate vicinity of the remains of the ancient castle, dating from around the 17th century, but of medieval origin of which only a few remains remain, and is inserted in the system of open castles in Lower Piedmont.
The church, first under the Diocese of Pavia and then of Acqui, passed to the Diocese of Asti in 1817 and was built between 1681 and 1688 to replace the previous church dedicated to Santa Maria della Spina, of which the sixteenth-century altarpiece remains right of the presbytery, attributed to the painter Giovanni Biancaro di Trino, representing the Madonna and Child holding a rose bush; the church preserves valuable eighteenth-century works such as the high altar in polychrome marble; the balustrade, the steps of the main altar and the first two side altars are the work of the marble worker Buscaglino di Olivola di Frassinello, dated 1728, while the frescoes are from the 19th century.
On the altar of the chapel of San Giuseppe there is a beautiful canvas representing the Nativity of Our Lord dated 1698, where in the lower right is reproduced the ancient coat of arms of the Community combined with that of the Trotti family, who owned the castle until 1837 , family of great patrons for Castelnuovo for which the unknown architect from the Milan area worked who took care of the church project; the master builders who took care of the erection of the church were the Gatti brothers of Castagnole Lanze. The workers mostly came from the Canton of Ticino: they were mainly craftsmen specialized in the construction of the vaults and the art of stucco.
On the altar of the chapel of San Giuseppe there is a beautiful canvas representing the Nativity of Our Lord dated 1698, where in the lower right is reproduced the ancient coat of arms of the Community combined with that of the Trotti family, who owned the castle until 1837 , family of great patrons for Castelnuovo for which the unknown architect from the Milan area worked who took care of the church project; the master builders who took care of the erection of the church were the Gatti brothers of Castagnole Lanze. The workers mostly came from the Canton of Ticino: they were mainly craftsmen specialized in the construction of the vaults and the art of stucco.
The baroque facade in exposed brick is punctuated by pilasters with capitals with elements of the composite and Ionic order supporting an entablature that separates the two superimposed orders and the semicircular pediment. The central access portal is underlined by a cross-vaulted prothyrum with columns on a high base with capital with elements of the Tuscan order and houses a door carved in walnut wood coeval with the church, by Besozzo and Bianchi, artists from the area. Lombard of Lake Maggiore, who also sculpted the pulpit and some furniture of the sacristy. Four statues dating back to 1943 depicting the Virtues Faith, Hope, Charity and Humility are visible within the niches.
Inside the church has a single nave with four side chapels dedicated to Sant'Anna, the Suffragio and the saints Antonio da Padova and Filippo Neri, the Madonna del Rosario and the aforementioned chapel of San Giuseppe. The stuccos of the chapels were the first decorative elements of the church: the work of the Bellotto brothers from Bellinzona, they were already finished before 1695. The barrel vault with lunettes is frescoed by the Turin priest Domenico Mentasti in 1881; the frescoes represent the Virgin in glory, Saint Stephen in glory and angels in adoration of the Blessed Sacrament. Along the walls of the nave are the paintings with the 14 stations of the Via Crucis painted in Rome in the second half of the 18th century and donated to the parish by Don Giuseppe Aluffi who was parish priest of Santa Maria in Trastevere for 18 years.
Noteworthy are still the eighteenth-century gilded and painted wooden statue of the Education of the Virgin Mary, by the sculptor Giovanni Battista Bonzanigo, in the chapel of Sant'Anna and the seventeenth-century gilded and painted wooden statue of the Madonna of the Rosary dating back to 1683, which was carried in procession on the occasion of the feast of Our Lady of the Rosary (October 7).