The main monumental buildings of Roman Trieste were built on the summit of San Giusto hill. In the second half of the 1st century A.D., the so-called Propylaeum and the civil Basilica in the original plan were built. The Propylaeum served as the monumental entrance to a sacred, enclosed area believed to have held the Capitoline temple. It consisted of two large lateral colonnaded structures with a stairway at the center. The dimensions of the visible remains give a good idea of the magnificence of the work. They are now partly incorporated in the bell tower of the cathedral, while the portion buried in the area in front, the stairway and the structure on the right are visible, proceeding downward into a tunnel that opens on the Orto Lapidario. The archaeological excavations carried out between 1929 and 1934 brought to light the remains of the civil Basilica, to the left of the Propylaeum. The structure consisted of three naves (measuring 88 x 23.5 meters) and, next to it on the side facing the sea, the Forum, or part of it. For the Romans, a Basilica was a large public building erected on the forum and used for sessions of court and the business of the merchants. The Forum was an open plaza, surrounded by public buildings, where Roman citizens met to discuss business.

In the Middle Ages, the bishopric, a monastery and the church of St. Sergio were erected on the site of the Roman Basilica, but no trace of them remains, while the Cathedral and Castle were built on the sides (the castle now houses the Lapidario Tergestino with the stone remains from the excavations of the city.