Duomo di San Gennaro
The Duomo is the main church of Naples. A wonderful Gothic cathedral built in the thirteenth century, it stands on the site of an earlier church dating from around 570 AD. It is dedicated to Naples' patron saint, San Gennaro. A vial of the saint's blood is brought out three times a year - on the first Saturday in May, September 19 and December 16 - and if it liquefies, all is well. If it doesn't... fears are held for the safety of Naples. Luckily, it nearly always liquefies.
The cathedral contains some excellent artworks including frescoes in the Chapel of the Treasure of San Gennaro, and some 4th century mosaics. To one side of the Duomo is the 4th century Basilica Santa Restituta, the oldest chapel in Naples, containing columns believed to be from the Temple of Apollo. Under here is an interesting archeological site tracing the Greek, Roman and early Christian city.
The nearest metro station is Piazza Cavour. Note that the archeological area is open on weekdays with the same hours as the cathedral, but it is closed weekends and religious holidays.