Ostia was the port of ancient Rome and a city in its own right. The currents and uneven bottom of the Tiber prevented Mediterranean shipping from going farther upstream, so merchandise was transferred to barges for the remainder of the trip. Ostia's fate was tied closely to that of the empire. At the peak of Rome's power, the city had 100,000 inhabitants -- hard to imagine looking at today's ruins. Ostia was important enough to have had a theater, numerous temples and baths, great patrician houses, and a large business complex.
Ostia flourished for about 8 centuries before it began to wither away. By Constantine's time (4th c. A.D.), the worm had turned. The barbarian sieges of Rome in the 5th century spelled the end of Ostia. Gradually, it became little more than a malaria bed, a buried ghost city that faded into history. A papal-sponsored commission launched a series of digs in the 19th century; however, the major work of unearthing was carried out under Mussolini's orders from 1938 to 1942 (the work had to stop because of the war). The city is only partially dug out today, but it's believed that all the chief monuments have been uncovered. Quite a few visible ruins have been unearthed, so this is no dusty field like the Circus Maximus.
These principal monuments are clearly labeled. The most important spot is Piazzale delle Corporazioni, an early version of Wall Street. Near the theater, this square contained nearly 75 corporations, the nature of their businesses identified by the patterns of preserved mosaics. Greek dramas were performed at the ancient theater, built in the early days of the empire. The classics are still aired here in summer (check with the tourist office for specific listings), but the theater as it looks today is the result of much rebuilding. Every town the size of Ostia had a forum, and during the excavations a number of pillars of the ancient Ostia Forum were uncovered. At one end is a 2nd-century B.C. temple honoring a trio of gods: Minerva, Jupiter, and Juno (little more than the basic foundation remains). Also of special interest are the ruins of Thermopolium, which was a bar; its name means "sale of hot drinks." The ruins of Capitolium and Forum remain; this was once the largest temple in Ostia, dating from the 2nd century A.D. A lot of the original brick remains, including a partial reconstruction of the altar. Of an insula, a block of multiparty apartments, Casa Diana remains, with its rooms arranged around an inner courtyard. There are perfect picnic spots beside fallen columns or near old temple walls.