The oldest church in Milan attests to the days when the city was the capital of the Western Roman Empire. This 4th-century, early-Christian structure has been rebuilt and altered many times over the centuries (its dome, the highest in Milan, is a 16th-c. embellishment), but it still retains the flavor of its roots in its octagonal floor plan and a few surviving remnants. These include 5th-century mosaics in the Cappella di Sant'Aquilino, which you enter from the atrium. A sarcophagus in the chapel is said to enshrine the remains of Galla Placidia, sister of Honorius, last emperor of Rome and wife of Ataulf, king of the Visigoths. Ironically, her mausoleum is one of the mosaic masterworks of Ravenna, and she is most likely buried in Rome, where she died. You'll be rewarded with a glimpse at even earlier history if you follow the stairs from behind the altar to a cryptlike room that contains what remains of a Roman amphitheater.