End at the Pantheon — the best-preserved building from the ancient world.
It has been in continuous use for nearly 2,000 years.
The reason it survived is that in AD 609 it was converted into a Christian church.
Everything that became a church had a chance of survival.
The dome is the marvel.
At 43.3 metres in diameter, it was the largest dome in the world for over 1,300 years.
Brunelleschi studied it obsessively before designing the dome of Florence Cathedral.
It remains the world's largest unreinforced concrete dome.
The oculus — the circular opening at the top — is 9 metres wide and has no glass.
Rain falls through it and drains away through small holes in the sloping floor.
On April 21st — the traditional date of Rome's founding — the sun shines directly through the oculus at noon and illuminates the entrance.
Whether that was deliberate, no one is quite sure.
Fun fact: the Pantheon was built by Emperor Hadrian around AD 125, but he inscribed the name of the original builder, Agrippa, on the front — a rare act of imperial modesty.
Inside are the tombs of Raphael and two Italian kings.
On your way here, stop at Piazza di Pietra — five minutes' walk away.
Eleven enormous Corinthian columns from the Temple of Hadrian, built by Antoninus Pius to honour the deified emperor in AD 145, are embedded into the wall of what is now the Rome Stock Exchange.
Standing next to them tells you something about Roman temple scale that no photograph can.
