Rome was founded as a network of seven villages perched on seven hills, So that Sabines, Latins and Etruscans could all benefit from the markets usefully arranged in the low-lying in between them. The Palatine was the central hillock, the Capitoline overlooked the marshy field of Mars and the Aventine was hard against the banks of the River Tiber. In all honesty the other four hills are not so distinct, just a series of interlinked spurs, but it has always been immensely propitious to have a unit of 7 in your foundation myth, like the very first civilization born in Mesopotamia, Sumeria. Rome doubled up by honoring a list of its first seven kings, beginning with Romulus (753-716), Numa (715-674), Tullus Hostilius (673-642), Ancus Marcius, Lucius Tarquinius Priscus and Servius Tullius and finishing with Tarquinius Superbus.
The power of Rome further spread the allure of a city being founded on seven hills, so that most of the great cities of the world—say, Moscow, Lisbon, Jerusalem, Istanbul or Barcelona—have a story of seven hills. Others which have hardly a hill at all, like Mumbai, are said to be founded on seven islands.”
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.