The parish church of St. Peter The Apostle in Zagarolo was built upon the ruins of another, smaller church which was demolished when it was no longer big enough to accommodate the needs of worshipers.
The old church was built in the 12th Century, this time on top of the ruins of a country aedicule. According to popular tradition, this had been built on the spot where St. Peter announced the Gospel.
The design of the new church was influenced by its central position, along the road that leads over the top of the hill where Zagarolo sits. The new building was therefore developed vertically, making it stand out from the surrounding urban area.
The dome is reminiscent of the other 17th-century domes in Zagarolo, St. Annuziata and St. Lawrence. This shows that its designer, architect Nicolo Michetti, wanted it to fit in with the existing architecture of the town. The first stone was laid on 30th June 1717. Michetti supervised the building work in person until Tsar Peter the Great summoned him to Russia in April 1718. Architects Ludovico Rusconi Sassi and Gerolamo Caccia from the Rospigliosi family took over building supervision.
The church was finished, complete with the coloured glass of the dome, on 10th October 1722 but wasn’t consecrated until 1st November 1725.
The church has a baroque style and an oval layout. The high altar, made from precious marble, and an altar piece by Giuseppe and Tommaso Chiari showing the “Delivery of the Keys” are particularly noteworthy.
At the sides of the high altar there are another 6 altars, three on the right and three on the left. The first altar on the left holds a painting of the “"Entombment of Christ" attributed to Ludovico Gimignani; at the second altar there is a painting by Domenico lacovacci of Zagarolo showing the Holy Virgin and Child, St Joseph and Anthony the Great in a contemplative pose.   A painting by Giovanni Agostino Ratti can be seen at the third altar depicting St Peter of Alcantara and St. Anthony of Padua worshipping Baby Jesus.
As for the altars to the right of the high altar, in the first we find a painting dedicated to The Immaculate Conception by an unknown artist; in the second there is a canvas by Gerolamo Pesci depicting "St. Dominic, Francis of Paola and Leonardo worshipping the Holy Trinity"; lastly, in the third altar, we find the only painting recovered from the pre-existing church dedicated to St. Cajetan and Anthony of Padua, the artist is unknown.
Beautiful false-marble stuccos from the 18th Century can be admired inside the church. Around 1917, the artist Giovan Battista Conti painted a fresco onto the dome which was previously completely white with pure gold ridge details. According to the elders, the church was brighter when the dome was white, before the frescoes were painted. 

Galleria fotografica