This is called a Megalopolis

"Megalopolis" is a term, not the name of a place. It means a very large & populated "metropolis" or city complex area that includes its surrounding metropolises (e.g., London, New York, Los Angeles). Both words are from Greek. A megalopolis is "formed" as a great city center grows & expands...

A megalopolis is a chain of metropolitan areas, such as those to be found along the American eastern seaboard, stretching from Boston to Washington D.C., or in Southern California (incorporating Los Angeles and San Diego).

A megalopolis is a term used to describe a heavily populated urban area that includes a number of connected cities or metropolitan regions. Examples include the Northeast megalopolis in the United States, which stretches from Boston to Washington, D.C., and the Taiheiyō Belt in Japan, which includes Tokyo, Yokohama, and other nearby cities.

Battle of Megalopolis happened in -331.