Chapter 7 of Earthopolis: A Biography of Our Urban Planet explores cities’ role as creators and creations of the global gunpowder arms race that ensued in the wake of the discovery of the interconnected World Ocean from 1500 on. While giant imperial capitals played a continuing role in this drama, an Age of Parity between the empires of Afro-Eurasia persisted as transformations in urban fortification systems spread throughout the world along with the invention of wall-breaking cannon and gunpowder battleships. Europeans took the lead in sparking this arms race – especially along the shores of the Atlantic Ocean ironically because of their initially disadvantaged position – though the arms race involved numerous acts of ocean-ward expansion and military resistance based in the polities of cities and smaller settlements on all continents. The result was the construction of hundreds of new fortified cities on all shores of the World Ocean and the merging of all pre-modern urban worlds into a truly planetary realm of city-enabled human action for the first time.