According to documents kept at the diocesan administration in the town of Casale Monferrato, the first church called San Pietro al Castello dates back to 1183. The parish was founded in 1701; its first priest was the Prior Giovanni Battista Broglia, who rebuilt the church in 1722.

The church of San Pietro al Castello is also reported in inventories of ecclesiastical assets carried out in 1563, 1612, 1630, 1688, 1701, and later.

The first known pastoral visit was the one made by the Bishop of Novara Girolamo Ragazzoni in 1577. The last pastoral visit, before the destruction of the aforementioned church, dates back to Bishop Avogadro in 1766.

The church of San Pietro al Castello was destroyed in the years following the French Revolution. In fact, Monsignor Alciati, as he inaugurated the new parish church built on the present site in 1805-1808, when the extant oratory of the Holy Trinity was also enlarged, noted that the bell tower was still incomplete, and hinted at "the old, destroyed parish church at the top of the hill." Beside the church was the cemetery, still functioning despite the church's desecreation.

The present Church, so scenically located and so rich in historical memory, was rebuilt in 1887.