This church is probably very ancient, but the structure we now admire shows unmistakable Romanesque features and dates back to the 11th - 12th and 13th century. As in the case of Saint Nicholas in Assenza, the first written evidence can be found in the papal bull of 1159. We find also references in the final wills of some well-off local residents like Domenico son of the deceased Ognibene in 1423, Antonio son of the deceased Moreto in 1427 and others.
This was the main church for the entire Brenzone territory until the Church of Saint John the Baptist replaced it as an autonomous parish at the beginning of the 1400s. St. Zeno was then turned into a simple prayer chapel. The present building is the result of three different construction phases.
The construction of the northern and eastern walls belongs to the first phase. The second phase took place during the 12th century. The apses, the two naves, and the southern wall were all completed during this time. This wall was later rebuilt after the enlargement of the church. The third phase occurred early in the 13th century and saw the erection of the bell tower and the reconstruction of the façade. This is the reason why the façade appears to have a single slope, but in reality the massive bell tower covers the second slope, the side aisle's falling roof. The main entrance is located on the central axis of the building and is made of pink marble jambs and architrave engraved with an encircled Greek cross.
A suspended vestibule and a tympanum with the image of Christ giving a blessing surmount the door; on the southern side one can see the imposing figure of Saint Christopher carrying the Child Jesus. Both paintings are from the same period as the frescoes on the walls of the interior.
On the eastern side we see the irregular and asymmetrical constructions of three apses. On the central apse there is one, original, splayed and arched window. On the northern wall, where the bell tower is located, there is evidence of a walled entrance, which once gave access to the tower. Lastly, the interior of the church is divided into two unequal naves by six small arches, fully curved, supported by alternating pillars. The Corinthian style capitals of the first column near the entrance and the third are Roman in origin, and were probably found in the area and recycled, like the slabs of pink marble.
These are perhaps remains of mortars once used as internal jambs of the main entrance. On the walls of the side aisle there are some late Romanesque frescoes with obvious Byzantine influences: The Angel and Zechariah; The Birth and Naming of John the Baptist; The Preaching of John the Baptist; The Beheading of John the Baptist. On the northern wall of the main nave we see: Cain and Abel, and The Fish. A fresco depicting The Apostles is barely visible along the basin of the main apse.