The church of San Bruno, now Co-Cathedral of Santa Maria Assunta, was built on a previous Roman foundation and over time has had many changes and renovations that also include a change of orientation of the building.
Around the tenth century, the village of Segni moved to a side of the hill and this made it necessary to give more significance to the church in a lower part of the town. The medieval church dates back to the tenth century and was upgraded by the best craftsmen of the time such as the Cosmati and the Vassalletto, families of artists in marble work. It was completed in 1185 by Bishop Pietro and had a single nave with 16 side chapels and a crypt.
The medieval church was damaged in the wars during the Sack of Rome in 1557 and in 1626 it was totally rebuilt by the architect Giovan Battista Roderi. The new building has a Greek cross shape with three naves, with three major and four minor chapels and a frescoed central dome. It was consecrated on 23 April 1684 by the bishop Francesco Maria Giannotti and dedicated to the Virgin Mary Assunta in Cielo. The interior is finely frescoed and decorated with paintings and other works of art and the altar and the white floor were introduced in 1875 on the occasion of the Jubilee.
A chapel is dedicated to San Bruno, with the relics of this saint who died in Segni and is the patron saint of the city. A large organ of 1855 completes the charm of the cathedral.
The white limestone façade is in neoclassical style and dates back to 1817.
The 11th-century medieval bell tower, 24 meters high, is divided into five levels with windows in the shape of single, double or triple mullions.
On the front of the church there are two clocks, corresponding to the entrances on the two side aisles, which date back to 1933.