After he made himself master of the Roman world, the emperor Constantine traveled to the site of Troy, in Asia Minor. He had decided to found a new capital for the eastern empire, and thought that Troy - the legendary origin of the first Romans - would have the required mythic cachet. Inside the ruined walls of the city, however, he heard a voice telling him, "The city of Priam belongs to the past. Leave it in the past."
Afterwards, he crossed the Bosphorus to the small Greek city of Byzantion, founded in the 7th century BCE by one Byzes, and initially settled by colonists from Athens and Megara. Here Constantine spent the night and dreamed, of a very old woman who became young again. On waking, he concluded that this would be the site of Nova Roma (New Rome), the place where a decrepit, elderly and dissipated world-city could rise again from its ruinous past. In 330 CE, following the dream, Constantine laid out the new city that would be known to the world, after him, as Constantinople, or, in its heyday, as simply The City.
After the Ottomans under the young Sultan Mehmed II broke down the previously impregnable walls of Constantinople in 1453, they renamed it Istanbul.
The story of the voice and the dream is less solidly evidenced than that of an earlier vision, followed by a dream, that led Constantine to march into battle under a Christian cross (the chi-ro symbol rather than the cross of Calvary) to victory over a rival emperor at the battle of the Milvian Bridge, before Rome, in 312 CE. I have discussed that sequence, which arguably led Rome (and thereby the West) to choose Christianity its established religion, in my Secret History of Dreaming. Maybe Constantine and his advisors simply had the smarts to appreciate the high strategic value of the site of the old Greek town commanding the trade routes between the Black Sea and the Mediterranean, and the crossings between Europe and Asia.
But, as I prepare to travel to The City that was once Constantinople later this month, I am tickled by the legend that it became a world city because of a dream. Again, se non è vero, è ben trovato; "if it's not true, it's well found." Come to think of it, the Ottoman empire, according to another (Turkish) legend also owed its origin to a dream, the dream of Osman, but this is a story for another time.
For the story of another city built on a dream, see my article on the dream of Grand Duke Gediminas, that led to the foundation of Vilnius in Lithuania. I have no doubt about the authenticity of this one.
Graphic: Constantine's Dream by Piero della Francesca (1466)