The Romans did not found London because they needed a city there. They founded it because they needed a crossing point on the Thames, and the crossing point became a city because trade and administration followed the military logic that had chosen the site. The settlement that grew up at the first substantial tidal ford on the Thames — approximately where London Bridge stands today — was called Londinium, and within a century of its founding it had become the administrative capital of the Roman province of Britannia and one of the most important commercial cities in the northwestern empire. Britain was at the edge of the known world; Londinium was a world city transplanted to the edge.