the Church, strongly desired by cardinal Branda Castiglioni, was built between 1422 and 1425 by the architects Alberto, John and Pietro Solari. Entirely in brick lombardo with elements in pietra serena, presents itself as a building in late-gothic style with a longitudinal plan is divided into three naves. The facade from the typical shape "a hut" and tripartite by buttresses is framed in the upper part by a characteristic trilobate terracotta arches. In the central part opens the entrance portal decorated with slender spiral turban-like and is surmounted by a carved lunette and dated 1428, with images of the Virgin Mary with the Child, Cardinal Branda Castiglioni, S. Ambrose, S. Clement, S. Lorenzo and S. Stefano.
Under the lunette were carved, within four niches, the four evangelists depicted with the heads of the evangelical symbols. Above the lunette there is a rose window in the stone, while the lateral parts of the facade have windows ogival with window sills in sandstone. On the left hand side overlooks the churchyard the building of the new school that allows access through the lovely inner garden, to the church of the Collegiate Church, the Baptistery and the little historical Museum (of the Collegiate Church).
Of particular interest are the frescoes by Masolino da Panicale, that decorate the vault of the apse of the church with a cycle of paintings dedicated to the life of the Virgin Mary, and frescoes by Paolo Schiavo and Lorenzo di Pietro called il Vecchietta with episodes of the life of the patron saints S. Lorenzo and S. Stefano.
Attached to the Complex, where once stood one of the towers of the ancient fortress, there is the Baptistery. Considered the jewel of Castiglione Olona, Masolino da Panicale, perhaps helped by Lorenzo di Pietro called il Vecchietta, there realized a masterpiece while he was frescoing the cycle with the stories of Saint John the Baptist, from the time of the announcement of the paternity of Zechariah to burial.