The large Collegiate Church of Our Lady of the Assumption is situated North West of the fortress of Castelnuovo di Porto. It is flanked by the Oratory of the Confraternity of the Name of Jesus (now a private residence and shops) to the east and the Cardinal's Palace to the west.
The church today has a single nave with four chapels on each side and a wide apse which leads into the sacresty and the choir. To the west, the bell tower shows 13th century architectural elements, including the base in large sandstone blocks and four orders of arches : the lower levels have been bricked up while the upper levels are marble and mullioned with foliage capitals. However, the Roman brick parament is redolent of anastylosis, refurbishing work which employs the original parts in different ways, in this case the marble elements. Following a lightning strike, restoration work took place in 1912, removing the tower's pyramidal tented roof.
The installation of the Commissioners of the Apostolic Camera in Castelnuovo in 1583 meant numerous diocesan synods were held in the Church and that many cardinals took interest in commissioning work for the church. Alderano Cibo restored the painting of the tribune and the altar in 1683, Flavio Ghigi expanded the church in 1689, Raniero Delci remodelled it in 1753, Giannantonio Guadagni built a chapel to St. Theresa in 1756 and Cardinal Lambruschini spent 1,000 scudi in renovation and decoration. The many works of art in the church include a notably well made Tryptych of the Saviour dating to 1501 in the Chapel of Degli Effetti.