from the 4th to 8th century (with later restoration) The church is mostly built in brickwork of different origins and colours, with parts of stone and white plastering. The current Romanesque church was begun around 1080. The
nave dates to about 1128, and the
rib vaults of the nave are from about 1140. The original edifice, like the great churches of Rome of the same epoch, belonged to the basilica type; it consisted of a central nave lighted from the
clerestory, two side
aisles, an
apse, and an
atrium. Investigations made in 1864 have established the fact that the nave and the aisles of the existing basilica correspond with those of the primitive church; the atrium, however, which dates from the 9th century, and two smaller apses, flanking a new central apse of greater depth than the original, was erected. The altar occupies about the same place as in the time of St. Ambrose, and the columns of the
ciborium over the altar appear never to have been disturbed; they still rest on the original pavement. In the following centuries, the edifice underwent several restorations and partial reconstructions, assuming the current appearance in the 12th century. The original
basilica plan was maintained, with an apse and two aisles, all with apses, and a
portico with arches supported by semi-columns and
pilasters preceding the entrance. The latter was used to house the catechumens who attended part of the Mass prior to receiving
baptism (this custom disappeared in the early 11th century); the portico, whose entrance has four blind arcades with an open one in the centre, was later used for civil and religious meetings.
Exterior The apex of the façade has two orders of loggias: the lower register has three arches of the same span, which join the slightly higher portico ones. The upper loggia was used by the bishops to bless the citizens. The portico's arcade is supported by pillars flanked by semi-columns. They have double
archivolts, while the portico's upper frame is decorated with
Lombard bands, which are repeated also on the façade. Thin
lesenes start from the pillars' centres, reaching the upper frame. The capitals are decorated by animals (lions, wild boars), and human figures (mostly heads, but also angels and others), as well as by vegetable or fantastic motifs of pre-Romanesque origin. Under the
narthex, between the central portal and the left aisle's portal, is the 15th-century
sarcophagus of
Pietro Candido Decembrio. The central portal is flanked by two multi-column pillars, and has an archivolt with decorative elements of
Sassanid inspiration. The basilica has two
bell towers. The right one, called
dei Monaci ("of the Monks"), is from the 9th century and has a severe appearance typical of defensive structures. The left and higher one dates from 1144, with the last two floors added in 1889. It was likely designed by the same architect of the Romanesque basilica since it contains the same decorative elements. There is also an octagonal tower over the east end of the nave.
Interior The basilica has a semi-circular
apse, and smaller, semi-circular chapels at the end of the aisles; there is no
transept. The third central span, on the left, houses the 12th-century Romanesque
ambon or pulpit, built on a pre-existing 9th century one; which utilizes as a base a 4th-century Roman sarcophagus, traditionally known as the
Sarcophagus of Stilicho. It has nine small columns with decorated capitals and friezes, featuring animal and human figures, as well as vegetable and fantastic motifs. The front of the ambon is decorated by two gilt copper reliefs, depicting the symbols of two evangelists, Saints Matthew (praying man) and John (eagle). Under the dome cladding, in the last span of the nave, is the
presbytery with, in its center, the high altar. This was realized in 824–859 by Volvinius. It features a golden
antependium with precious stones on both sides. The altar is surmounted by a contemporary
ciborium, commissioned by archbishop of Milan
Angilbert II, whence its common name. It has four columns in
red porphyry and has, on each side, bar-reliefs depicting
Christ with Sts Peter and Paulus (front side),
St. Ambrose Receives the Homage from Two Monks, at the Presence of Sts Gervasius and Protasus (rear side),
St Benedict Receives the Homage from Two monks (left side), and
St. Scholastica Receives the Homage from Two Nuns (right side). The apse displays an early 13th-century
mosaic, depicting
Christ Pantokrator with Sts Gervasus and Protasus, and at the sides,
Scenes from the Life of St. Ambrose. The apse mosaic was heavily restored after damage during
Second World War. The oratory of ''San Vittore in Ciel d'Oro
was originally a free-standing chapel, putatively begun in the 4th century, and thus pre-dating the church, commissioned by bishop Maternus to hold the relics of San Vittore''. Bishop Ambrose supposedly buried his brother, San Satiro, in the chapel. The mosaics on the walls and ceiling were created in the 5th century; these include one of the earliest portraits of St Ambrose. The gilded dome ceiling has a central portrait of the patron saint. The church also houses the tomb of Emperor
Louis II, who died in
Lombardy in 875. The
crypt, located under the high altar, was built in the 9th century to house the remains of three saints venerated here: Ambrose, Gervasus and Protasus. The remains of the saints were already in a crypt in the area, although their position has since been lost over the centuries. In the 9th century bishop, Angilbert found them and had them put in a single
porphyry sarcophagus. The current appearance of the crypt dates from the 18th-century restoration commissioned by cardinal
Benedetto Erba Odescalchi and to others from the following century, in which the bodies of the three saints were moved to a silver urn in a space under the ciborium. ==Myth==