Perhaps the most extraordinary thing about the Eternal City is its historical continuity. The Ponte Fabricio, which spans the Tiber to the Isola Tiberina, dates from 62 B.C. and is still in daily use. And 2,000 years after it was built, the Pantheon still has the world’s largest unreinforced concrete dome. One of the best introductions to the city remains “The Companion Guide to Rome,” by Georgina Masson, first published in 1965. We have long enjoyed following its detailed walking itineraries. Rome is particularly beautiful during the so-called “ottobrate,” from mid-September to the end of October. High summer is hot and disagreeable, and the Romans themselves leave for the seaside.