One of the earliest churches in Milan is the Basilica of Saint Ambrose, or Basilica collegiata abbaziale prepositurale di Sant'Ambrogio in Italian.

It was constructed between 379 and 386 by Saint Ambrose, the Milanese bishop, on the site where Christians who died during the Roman persecutions had previously been buried. Ambrose himself wished to place all the relics of the martyrs Satyrus and Victor, Nabor, Vital, Felix, Valeria, Gervasius, and Protasius in it. For this reason, it was consecrated to the martyrs and given the name Basilica Martyrum. This church, which has since been named for him, also served as Ambrose's grave.

Bishop Angibert II (824-860) made significant changes in the ninth century. A massive mosaic depicting the Redeemer seated between the martyrs Protasius and Gervase, with the archangels Michael and Gabriel, as well as two scenes from the life of St. Ambrose, was added as the apse, which was preceded by a barrel vault.

The early Christian ciborium received four stuccoed tympani in the tenth century. The altar of St. Ambrose, a masterwork of Carolingian gold, silver, precious stone, and enamel work, was placed beneath the ciborium.

Between 1088 and 1099, the basilica underwent its final transformation thanks to Bishop Anselm's initiative to rebuild it using Romanesque style architecture. A quadriportico and the three naves without transepts and their three corresponding apses were kept.

Near the end of the 12th century, the tiburium was erected, but it fell apart in 1196. After afterwards, it was reconstructed with its distinctive exterior design, which has arched galleries on two stacked floors.

The poor quality, locally sourced building materials used are primarily white plaster, stone, and various colored bricks. The same materials were used to construct the nearby buildings. The decision to forego using more expensive materials imported from the East is the result of modern sensibility, which no longer divides matter and soul.

The new church meticulously followed the original's layout, which included three naves, apses, and a quadriportico. The basilica's longitudinal interior layout matches the portico's size (excluding the apses).

The church's long and low façade, which is typical of other rural buildings, reflects the link to the land that was a major inspiration for the design of the building. It has two loggias that are superimposed. The lower one connects the portico's inner perimeter and features three equal arches. Contrarily, the top one contains five arches whose heights rise in accordance with the slopes' profile. It contains Lombard bands as well.

The groin vault was used to create an interior with a transalpine structure. The matronei are supported by smaller pillars that support the side aisle vaults, which have intercolumniations that are half the size of an intercolumniation in the central nave. These latter take up the entire clerestory space.

At the side of the church, the quadriportico served as a gathering place for catechumens. The quadriportico, however, lost its primary purpose with the practice of child baptism at the start of the 11th century. In the end, it served as a venue for outdoor gatherings. The archbishop bestows his benediction on the people of the city from the upper loggia of the façade.

There are four walls around the portico (hence the name). It has arches inside that are enclosed by half-columns and are supported by pillars. The portico's various components can all be identified by their colors. Similar to those on the façade, Lombard bands support the cornices.

The bell tower to the right, often referred to as the monks' bell tower, was built in the eighth century and has the stern look that defense towers are known for. The one on the left, which is taller and is called the canons', was built in 1144. Given that it utilizes the same vertical principles as the quadriportico, its conception can presumably be assigned to the same architect who created the basilica. In 1889, the upper two floors were constructed. Additionally displaying influences from outside the Alps are the bell towers.