The building of the basilica started in 379 A.D. on bishop Ambrose will, in an area where were originally placed votive chapels, cemeteries and little churches : initially it was called Basilica Martyrum, but when Ambrose died it adopted his name. It's built with red bricks made locally, its plan is longitudinal, it has cross vaults held up by massive pillars with decorated capitals and by wide round arches. Thick walls, solid structure, imposing facade give to this basilica a big sense of strenght and stability, solemnity and calm, which are its most charming aspects.
On the left side of the basilica there is a roman column, brought here during the Middle Ages, which has got two big regular holes and a long scratch. The legend tells that they have been made by the devil, angry because Ambrose had won him. For that reason in ancient times the faithful Christians used to hug the column in sign of devotion.
During the centuries the basilica underwent various changes, started in the VIII century when the Benedictine, the monks who lived in the next monastery, altered the apse. More changes were introducted by bishops Angilberto and Ansperto,that in particular added the monumental porch at the entrance. The Basilica was further alterated in the XI century (by the Commune) and between seventeenth and eighteenth century when some baroque structures were added and then removed in 1857).
For a big part of its history, the Basilica was marked out by the rivalries between the Benedictines and the external canonics; in particular they challenged each others with the two bell towers. In fact the monks had built in the VIII century the first bell tower, the one on the right side, that they used for their monastic duties. But in the XII century the canonics built another one, on the left side, taller and destinated to civil duties.
The Basilica is introducted by a huge and impressive atrium, containing tombstones, graves, fragments of roman inscriptions. The columns, in classic style, have capitals adorned with allegoric representations, animals, medieval monsters, all different.
In the church there are a nave and two side aisles and many art works of big value.
In the right aisle there is the important chapel named San Vittore in Ciel d'Oro, containing the mortal remains of San Vittore, Nabore and Felice. There are also wonderful mosaics of the V century made in gold tesseras: in one of them is represented Saint Ambrose in a very realistic and characterized way.
In the left aisle there is a marble pulpit decorated with sculptures and in particular with the representation of a man and an eagle made in gilt copper. Moreover, there is a part of the
original floor still visible and next to it a roman column on whose top there is a bronze snake: the legend says that on the Doomsday it will fall from the column and direct to the Josafat valley.
But the most valuable characteristic is certainly the Gold Altar in the middle of the apse: it is surmounted by a cuspidate Ciborium of the IX century and it is decorated with the representations of the Saints Michael, Gabriel and twelve scenes of Saint Ambrose's life. This Altar is exceptionally precious because it is all gilt and studded with more than four thousand pearls and corals and about a hundred emeralds and sapphires.
Finally, the last interesting aspect of the basilica is the Crypt, a little chapel situated under the apse, where, kept in a silver and crystal shrine, there are the relics of Saint Ambrose and of the two martyrs Gervasius and Protasius, dressed with precious drapes and mantels.
In ancient times the abbots of Saint Ambrose were so important and influential that a Longobard feudatory, that owned some lands around the village of Campione, left them to the basilica: so, the area around this village are still part of the italian territory, though they are reachable only by passing trough Switzerland.