Behind the cathedral, Civitavecchia's centro storico begins, and is actually quite a nice place to walk around. True, there aren't any amazing must-sees, and fans of architecture will definitely get more of a thrill in 1001 other Italian towns, but Civitavecchia's centro storico is almost completely free of tourists, which in my book is a very big attraction. The pretty square of Piazza Leandra is the nicest part, all cobbles and fountains and green shutters and gossipping women and laundry hung out to dry.
A couple of blocks in from the old city walls, Civitavecchia's cathedral overlooks a concrete expanse of car park, and the impressive structure looks rather silly with some of Italy's less charming apartment blocks around it.
Civitavecchia is a busy port, attracting up to a dozen cruise ships and numerous ferries each day in summer, so there is a constant stream of ship passengers heading right out of the port, along the seafront to the train station for the first train out to Rome. Go against the herd and turn left instead, walk between the old city walls and the harbour, and eventually you'll come to a gate. A sparkling white gate, in fact, which has probably had a bit of work done...but it is still quite a sight. This is the Livorno gate, or Porta Livorno, and nowadays doesn't really serve much of a purpose...the steps take you to a pizzeria in a modern buidling. The old city must have shrunk in the last century, probably to do with the heavy bombing in the WWII, and you now have to walk inland a couple of blocks before anything else historical reveals itself.
There's not much more to the centro storico, just a few streets with old buildings and the odd church...nothing too exciting, but nice to walk around if you are sick of crowded touristy towns.