In considering the urban planning of the Gallo-Roman city, we see that topographical considerations dictated the dominant orientation. There was a need for a road that took the shortest possible route across the river and its marshy banks. The existence of a series of mounds and small islands that formed natural bridge piles also seems to have played a part in determining what the Romans referred to as the cardo maximus, i.e. the principal north-south road within an urban area.
In Lutetia, this route was the junction between the left bank, the set of islands that formed the Île de la Cité, and the right bank. Dendrochronological study of wooden poles found beneath the original road date its construction to 4 CE at the earliest, more than fifty years after the Roman conquest. There is no evidence that this road existed under the Gauls, or even that there was another river crossing at Paris.
In fact, Lutetia's cardo was more than just the principal axis of the Roman town-it was the urban portion of a route that crossed the Seine. On both banks, other major roads joined up with it to take advantage of this crossing. Lutetia thus appears to have been a "bridge town" that took its place in the great Gallo-Roman road network. This is attested to by archaeological sources and by such documents as the Peutinger Table