Image by Simon Montgomery Getty Images
Although it's now the cathedral's baptistery, this wonderfully intact building was originally an ancient Roman temple dedicated to the king of the gods. It still has its original barrel-vaulted ceiling and decorative frieze, although a striking bronze statue of St John the Baptist by Ivan Meštrović now fills the spot where Jupiter once stood. The font is made from 13th-century carved stones recycled from the cathedral's rood screen.
Of the columns that once supported a porch, only one remains. The black-granite sphinx guarding the entrance was already ancient when the Romans dragged it from Egypt in the 3rd century. It was defaced (literally) by early Christians, who considered it a pagan icon.