One of Lazio's prize sights, the ruins of ancient Rome's seaport are wonderfully complete, like a smaller version of Pompeii. Highlights include the Terme di Nettuno (Baths of Neptune), a steeply stacked amphitheatre, and an ancient cafe, complete with a bar and traces of the original menu frescoed on the wall.
Note that the site is pretty large and you'll need a few hours to do it justice. Also, it gets busy at weekends, but is much quieter on weekdays.
Near the entrance, Porta Romana gives onto the Decumanus Maximus, the site's central strip, which runs over 1km to Porta Marina, the city's original sea-facing gate.
On the Decumanus, the Terme di Nettuno is a must-see. This baths complex, one of 20 that originally stood in town, dates to the 2nd century and boasts some superb mosaics, including one of Neptune driving his sea-horse chariot. In the centre of the complex are the remains of an arcaded Palestra (gym).
Next to the terme is the Teatro, an amphitheatre originally built at the end of the 1st century BC by Agrippa and later enlarged to hold 4000 people.
The grassy area behind the amphitheatre is the Piazzale delle Corporazioni (Forum of the Corporations), home to the offices of Ostia's merchant guilds. The mosaics that line the perimeter – ships, dolphins, a lighthouse, an elephant – are thought to represent the businesses housed on the square: ships and dolphins indicated shipping agencies, while the elephant probably referred to a business involved in the ivory trade.
The Forum, Ostia's main square, is overlooked by what remains of the Capitolium, a temple built by Hadrian and dedicated to Jupiter, Juno and Minerva.
Nearby is another highlight: the Thermopolium, an ancient cafe, complete with a bar, frescoed menu, kitchen and small courtyard where customers would have relaxed by a fountain. Just to the north of the Thermopolium are two of the site's so-called case decorate. These frescoed houses are off-limits to unaccompanied visitors but can be visited on a guided tour at 10.30am each Sunday (plus 2.30pm on the first Sunday of the month) – book a place by emailing email@example.com.
Over on the other side of the Decumanus are the remains of the 2nd-century Terme del Foro, originally the city's largest baths complex. Here, in the forica (public toilet), you can see 20 well-preserved latrines set sociably in a long stone bench.
For more modern facilities, there's a cafeteria-bar complex with toilets and a gift shop to the north of the Decumanus (head up Via dei Molini). Also at this complex is a small museum displaying statues and sarcophagi excavated at the site.