Rome

Paul wrote to the Christians here before he ever saw the city, and arrived as a prisoner. Peter, tradition holds, was crucified in Nero's circus on the Vatican hill. For two and a half centuries the faith in Rome was illegal, and its dead were laid in the catacombs along the consular roads.

With Constantine everything inverted. Basilicas rose over the apostolic graves — St Peter's, St Paul Outside the Walls, St John Lateran — and Rome became the destination of the Via Francigena and the terminus of medieval Europe's pilgrim traffic.

Walking the city, the two eras sit on top of each other: an imperial forum, a titular church with a fourth-century floor, a baroque façade, and beneath it all the tufa galleries where the earliest Roman Christians buried their neighbours.