These are sixteen columns and a section of the relative architrave which were originally part of a temple or perhaps a public baths complex. They date back to the end of the 2nd century AD, and were transported to this site in the 4th century AD to form the front side of a four-sided portico in front of the basilica of San Lorenzo Maggiore. (Tram: 2,3; Bus: 94).
This church (7:30AM-6:45PM) has obviously changed a lot from the 4th century AD, when it was the largest centrally planned building in the Western world.
It certainly did not have, stuck in front of it, the monument to Constantine, a contemporary replica which is supposed to honor the glory of Rome, yet fails to do so. The reconstruction of the basilica— between 1573 and 1619, to a brilliant project by Martino Bassi — nonetheless re-mained fairly faithful to the original Early Christian plan, a quite rare occurrence in the history of Western architecture.