If you cross a short hall from the Basilica's façade, you'll find yourself in a very particular space inside. Despite the renovations, the interior still has the same appearance it did during the Roman Empire, as can be seen in the four large recesses that surround the central space. The building materials were partly recycled from a nearby Roman amphitheater, of which you can still see some remains in the archaeological park near the Basilica.
The main thing that differentiates San Lorenzo from the other churches built in Milan in the same period is that it has a central plan, which means it doesn't have any naves. This is basically a square, domed room surrounded by a wide corridor. The upper gallery was reserved for celebrities or women, and for this reason was called the "matroneum", or women's gallery.
There are several chapels around the central building. The most important is the first on the right dedicated to Saint Aquilino. I'd especially like to point out the splendid Roman portal in marble with its vibrant chariot race, reminding us that the Roman circus where the races were held was once located nearby.
You can see fragments of mosaics on the walls, and there are still traces of the monumental figures of the tribes of Israel's twelve founders in the atrium, while two mosaics are conserved in the chapel, one of which is completely intact. Can you see how realistic those four horses are, panting and turning around?!
As you may have noticed, the Basilica also houses many paintings from different periods. One of the most unique ones is the fresco that you'll see at the end of your tour here: it is one of the oldest replicas of Leonardo's Last Supper, and was done by an anonymous Lombard painter a few years after the masterpiece was completed.
FUN FACT: in the second half of the 1500s, a fire severely damaged the Basilica. The restorations began and then stopped almost immediately afterwards for a lack of funds. Then the Archbishop of Milan predicted that there would be a miracle. And the miracle came a year later, when a seriously ill woman was suddenly cured while looking at a painted Madonna. The event triggered the generosity of the Milanese, and the restoration work resumed.