Known as the City of Popes, the walled city of Avignon was home to seven popes throughout the 14th century, who protected their papal territory by building immense fortifications.

"When in Rome, do as the Romans." That classic advice leads to a wealth of options: Should you follow Augustus and Caesar, whose Rome is filled with spectacular ancient structures?

For a bird's-eye view of London, take a spin on the London Eye, the Ferris wheel overlooking the Thames. Don't miss landmarks like Big Ben, the Tower of London, St. Paul's Cathedral and the Globe Theatre.