The Basilica of San Simpliciano was located along the road that connected Milan to Como (currently Corso Garibaldi) and, through the Spluga Pass, to the Rhineland. According to Benzone of Alexandria, who wrote a booklet dedicated to the city of Milan in the 13th century, the foundation of the basilica is attributed to St. Ambrose. He would have dedicated it to Mary and the holy Virgins (basilica Virginum), perhaps to reaffirm the dogma of the virginity of Mary, which was questioned by a heresy and reaffirmed in a synod convened by Ambrose himself in 393 AD.
However, some scholars believe that the construction was initiated by Ambrose but completed only by his successor, Bishop Simpliciano, to whom it was later dedicated in Lombard times. Simpliciano also allegedly deposited the relics of three Christian preachers, Martirio, Sisinnio, and Alessandro, who were sent by St. Ambrose to the Val di Non to preach Christ and were martyred there. In 1582, St. Charles Borromeo found their remains, along with those of St. Simpliciano, under the altar.
The architectural renovations that affected the basilica between the 11th and 12th centuries concealed its ancient origins for several centuries. It was Edoardo Arslan who revealed in 1944 that the Romanesque church actually preserved the paleochristian walls to a height of 22 meters. The restorations brought to light the large original windows that opened in the walls and that had been blocked up in more recent times. It was found that the paleochristian building underwent the first radical change at the beginning of the 7th century AD.
The primitive basilica had a Latin cross plan and a single nave (56.70 x 21.70 meters) closed by an apse. The roof was made of wood and the floor decorated in “opus sectile”. The walls, still visible, are built with horizontally arranged rows of bricks alternating with bands of smaller bricks arranged in a herringbone pattern. The outer walls of the nave have two orders of blind arches within which large windows were carved out, which are still recognizable; the arches of the transept, on the other hand, have only one order and are equipped with two superimposed windows.