From an artistic point of view, the Cathedral is a significant example of the transition from Baroque architectural forms to those of 18th-century Neoclassicism. The imposing and majestic façade (36.75 m wide, 38 m high) is divided by semi-columns and pilasters with Corinthian capitals; it is adorned by an elegant balustrade and seven marble statues depicting the patron saints Luigi Gonzaga. The spacious, bright and striking interior for the balance of the simple and grandiose neoclassical lines, has three naves divided by six gigantic pillars with rich capitals. In the vault there are three minor domes and a central one. The titular saints of the church are the martyred Saints Nazario and Celso, depicted together with St. Louis in the altarpiece of the high altar, the work of Luigi Sigurtà. There are numerous testimonies to the Castiglionese saint in this parish church. On 20th April 1568, Marta Tana and Ferrante Gonzaga had their first-born son Luigi baptised here; on 22nd July 1580, from the hands of Cardinal San Carlo Borromeo, Luigi received his First Communion here; one of his relics is placed in the third chapel on the right of Castiglione delle Stiviere; finally, at the foot of the presbytery, his mother, Donna Marta Tana of Santena, a woman witness to great Christian virtue, is buried here.

The church of Ss. Nazario and Celso took on further importance in 1607, when Prince Francesco Gonzaga, thanks to his diplomatic services to the Holy See, obtained the nomination of the parish church as Collegiate Church, i.e. 'quasi episcopalis' seat, in which the Abbot presided with six Canons. In 1762, reconstruction of the building, which had become insufficient for the needs of the community, was begun. Based on a design by Giambattista Groppi, the remains of the castle, destroyed by the French in 1706, were used, moved to this hill by long passages of volunteers from Castiglionese. Thus the place where St. Louis had spent his early childhood with his family, 'his home', was used to build the 'House of the Community'. With the advent of Josephinism, the church lost its canonical privileges and on the arrival of Napoleon, it was despoiled of several of its possessions; the same happened to all the other Italian churches when the 'uom fatal' passed through.

1. Apse. High Altarpiece with St. Louis and Saints Nazario and Celso, by the Castiglionese artist Luigi Sigurtà (1777). Chancel of the abbey chapter, in the apse, 25 inlaid walnut stalls, work attributed to the workshop of the Ceratelli, renowned craftsmen who worked in Castiglione during the 18th century. 
2. Chapel of the Crucifix. Of this chapel, the altar is admirable, rich in marble and stucco, and above all the fine workmanship of the marble inlay of the antependium. 
3. Chapel of the Virgin. In Neo-Dausian style, rich in polychrome marble, on which the white of the mensa, base and columns predominates (late 18th century). The wooden statue of Our Lady of the Rosary (1929) is the work of Brescian sculptor Angelo Righetti. Next to the altar are two statues, of David and Isaiah, sculpted by Carlo Romano in 1865. 
4. Chapel of St. Anthony of Padua. Under the altarpiece a plate, laminated in gold indicates the place where the relic of St. Louis is kept, of which the decree of authenticity is kept in the archives. 
5. Chapel of St. Anthony Abbot and St. Pio of Pietralcina 

6. Chapel of St Rosalie. Like the altarpiece, this is a valuable painting by Pietro Novelli, a famous painter from Monreale, made for the plague that struck our land in 1630. In it, Saint Rosalie "Eris in peste patrona" intercedes with the Holy Trinity for the principality of Castiglione, which she points to with the forefinger of her left hand. Further down, a Castiglionese woman (with the symbolic Aloysian lily in her hands) pleads with her from the desolate earthly reality of suffering and death. Above the altar is a beautiful 16th century tabernacle made of carved and gilded wood. 
7. Altar of the Sacred Heart. Work of an unknown 17th-century artist with an antependium in polychrome marble depicting a wreath gathering lilies and roses. Of historical value is the plaque commemorating the erection of the church as a Collegiate Church on 28 July 1607. 
8. Chapel of the Blessed Sacrament. Equipped with a sumptuous altar for mami and metalwork, which begins the 17th-century canvas depicting the institution of the Eucharist. The work has a certain imprint of realism and Caravaggesque taste, due to the play of light and shadow, the study of perspective and the refined use of colour. 
9. Chapel of St. Joseph

10. Chapel of St Catherine of Alexandria. A valuable painting, dated 1606 - by the Bolognese Camillo Procaccini (1551-1629), depicts the mystical wedding of the saint with the Child Jesus. The composition of the work is still in 16th century taste. 
11. Baptistery. It is bordered by two balustrades, previously placed to close the presbytery, with bas-reliefs by Giuseppe Brigoni (1935), depicting the martyrdom of Saints Nazario and Celso and the First Communion of St. Luigi Gonzaga. The wooden sculpture of Christ on the Cross is by Castiglionese sculptor Aldo Rossi. 
12. Sacristy. Octagonal in plan, spacious, with wooden furniture from the school of Ceratelli perimetering the space; a large central table one metre high for the preparation of the various liturgies, on the vault four well-made ovals recently restored, the wooden Crucifix from the school of Mantegna and some paintings by Abati. 
13. Cloister recently used as a space for reflection and contemplation of the stages of the Via Lucis, effectively illustrated on 14 ceramic panels by the Bertolasi artists from Poggio Rusco (MN).