St. Leonardo's Collegiata is the main church in St. Casciano. It was raised between the XII and the XIV century as a parish church, but both its appearance and its spiritual function changed during the years because of the influence of the late Renaissance period.

Originally, the church was run by the "pensionieri" of St. Guglielmo, namely four priests depending on the abbey of Acerona where the Guglielmite monks lived. Thanks to their presence the church was dedicated to St. Guglielmo, a saint worshipped by the friars from Acerona. Parallel to the growth of the thermal activity, the church of St. Leonardo was a destination of well-known clergymen coming from all over Europe to stay in St. Casciano and take a cure.
The church's fame led it to be raised to the status of Insigne Collegiata in 1618. Therefore, the four "pensionieri" were substituted by seven clergymen, one of whom was the archpriest at the head of the other six canons and they all were appointed by the inhabitants of St. Casciano. Moreover, five "benefizi" were instituted and the most important families of the town had the duty to nominate them.
The Collegiata was subjected to many alterations during the years. Originally, the entrance door was on the side where today there is the Oratory of St. Antonio. In effect, during some renovations, the presence of the old portal was discovered.
At the beginning, the church was conceived following the Gothic style, characterized by the opulence of some parts in carved travertine. The current architectural features were probably created in the XVI century during the construction of the Oratory, built to celebrate the birth of the St. Anthony's Fraternity. The bell tower, instead, dates to 1606.