The Basilica of San Simpliciano, whose façade is set back from Corso Garibaldi but is nonetheless visible from that street, is one of the four churches built by bishop Ambrose.
The arches framing the doors show that there was once a portico annexed to the church. The squat tower shows Late Renaissance two-light windows. The tower itself was actually lowered by about 25 metres by Ferrante Gonzaga, a fate shared by many other important buildings near the Castle.
San Simpliciano became famous during the battle of Legnano (1176), when the “Lombard League” army beat the forces of Emperor Frederick Barbarossa. The stained glass on the façade illustrates scenes of the Carroccio, a sort of cart that provided a focus for the Lombard League's army during combat.
Giulio Aluisetti performed a highly unsatisfactory restoration in 1841, when he plastered and simplified the columns in the nave. More recent restoration work has revealed part of the original Palaeochristian structures, and has concentrated on restoring the church's original Romanesque characteristics.
The church became famous after the victory of Legnano, following the circulation of a legend according to which, on 29 May 1176, the souls of the three martyrs buried here (Martirio, Sisinio and Alessandro) took visible shape in the form of doves, which flew to the battlefield and landed on the top of the Carroccio, giving renewed enthusiasm to the Lombard League warriors. The League went on to defeat Frederick Barbarossa's army.
Don Ferrante Gonzaga ordered the bell-tower to be lowered by 25 metres, because he found it irritating that it was higher than the Castle in which he lived.
For a considerable time, Francesco Petrarca was a guest in the monastery attached to the church.