The port that Rome does not have
Rome is not a coastal city, and Civitavecchia is the answer it gave itself. The port was dug by Trajan around 107 AD, when the settlement was called Centumcellae, and it has served the capital ever since. Today it is the busiest cruise port in Italy and one of the largest in the Mediterranean, which is why most visitors meet the town for exactly as long as it takes to leave it.
The number that governs everything here is 80 km. That is the distance to Rome, an hour and a quarter each way by road, and it is the figure to plan a day around, because a ship's all-aboard time does not move.
If you have a day and you want Rome
Count the return journey first, then decide how much of the city fits in the middle. Two and a half hours of travel out of a nine hour call leaves six and a half, which is enough for the Colosseum and the centre, or for the Vatican, and not comfortably for both.
The train reaches Roma Termini or Roma San Pietro in about 70 minutes, but the station is a shuttle ride from the berth and the timetable, not you, decides the departure. A private transfer removes both variables: it collects you at the terminal, drops you at a point agreed inside the city, and brings you back with a margin on the all-aboard time. Whichever you choose, the constraint is the same and it is not negotiable.
If you stay in town
The Forte Michelangelo stands over the harbour, begun in 1508 to a design attributed to Bramante and finished decades later, with the upper tower traditionally credited to Michelangelo, whose name it kept. It is best seen from the water as the ship comes in.
Along the seafront runs the Pirgo, a beach and a raised walkway ending in a platform over the sea. The original was the smartest bathing establishment on the Lazio coast between the late nineteenth century and the early twentieth, built entirely of wood; it was destroyed and rebuilt in a modern form in 1997. Inland, a few kilometres out, the Terme Taurine are a Roman thermal complex on a hillside with a view back over the coast, and they are almost always empty.
The town centre was heavily bombed in 1943 and rebuilt afterwards, so it does not read as an old port: what is old here is the harbour itself, not the streets around it.
Beyond Rome
Not every call has to end in the capital. Tarquinia, half an hour north, holds the painted Etruscan tombs that are a UNESCO site and a rarer sight than anything in Rome. Cerveteri, closer, has the Etruscan necropolis built like a town of the dead. Both are short journeys from the port and neither is on a railway line that makes them easy.