Rome has 4 major basilicas: St. Peter, St. Paul outside the Walls, St. John in Lateran and St. Mary Major. They’re all open daily . Free admission.
Other important churches:
St Clement: a church on three levels. An upper church dating back to 12th century which was built over the previous basilica of the 4th which at his own turn was built over some Roman houses dating back to the 1st century AD. Exploring the two underground levels is like travelling through Rome’s history. Admission fee to the underground levels.
St Peter in Chains: it was built, according to tradition, to house the chains of the apostle Peter. Mostly reknown for the stunning Moses by Michelangelo. Free admission
Chiesa del Gesù: a superb baroque church where is buried St. Ignatius of Loyola. Free admission
St. Ignatius: dedicated to the Jesuit saint this beautiful baroque church is famous for its trompe l’oeil dome. Free admission.
The Catacombs: early christian underground cemeteries. These subterranean cemeteries consist of a network of galleries dug out in the tufa soil, strectching for several miles and built on different levels. There were many types of burials: from the cheap slots in the wall to the beautifully painted niches and cubicula of the wealthy families. There are four major catacombs areas open to the public and they all offer guided tours included in the ticket price: the catacombs of Saint Sebastian, St. Calixstus and Saint Domitilla are on the Appian way area, while the catacombs of Saint Priscilla are on the Salaria way, on the northern side of the city. Admission fee