Built at the end of the fourth century, San Lorenzo is one of the oldest centrally planned churches, and may have been the chapel of the imperial Roman palace. Fires all but destroyed it in the 11th and 12th centuries, but it was rebuilt on the exact lines of the original model. When the cupola collapsed in 1573, the new dome - the tallest in Milan and a far cry from the original - outraged the locals.
On the backs of the two great arches that flank the main altar, columns were placed upside down to symbolise the Christian religion rising up out of the ruins of paganism. To the right, the octagonal fourth-century Cappella di Sant'Aquilino may have been an imperial mausoleum. Legend has it that a group of porters discovered St Aquiline's corpse in a ditch; taking it to the Duomo, they got lost in the fog and ended up in San Lorenzo. Thus his remains are still here, in a glass coffin on top of the altar; he also became the patron saint of porters. On the walls of the cappella are fragments of the late fourth-century mosaics that once covered the entire chapel. Behind the altar, descend the stairs to the passage under the church, where stones from pre-existing Roman structures used in the construction of San Lorenzo can be seen.
Outside the church stand 16 Corinthian columns from the second and third centuries. They were moved here from an unidentified pagan temple some time in the fourth century, and topped with pieces of architrave, only some of which date from the same period. The 17th-century wings flanking the entrance to San Lorenzo were designed to link the columns to the church in a sort of pseudo-ancient atrium. In the centre, a bronze statue of Emperor Constantine is a copy of one in Rome - a reminder of his Edict of Milan (313), which put an end to the state persecution of Christians.