Indeed from time immemorial the clan of the Romilii, once the chief probably of all the Roman clans, was settled in this very quarter; the Janiculum formed a part of the city itself, and Ostia was a burgess colony or, in other words, a suburb.
The Tiber and Its Traffic This cannot have been the result of mere accident.

The Tiber was the natural highway for the traffic of Latium; and its mouth, on a coast scantily provided with harbours, became necessarily the anchorage of seafarers.

Moreover, the Tiber formed from very ancient times the frontier defence of the Latin stock against their northern neighbours.

There was no place better fitted for an emporium of the Latin river and sea traffic, and for a maritime frontier fortress of Latium, than Rome.

It combined the advantages of a strong position and of immediate vicinity to the river; it commanded both banks of the stream down to its mouth; it was so situated as to be equally convenient for the river navigator descending the Tiber or the Anio, and for the seafarer with vessels of so moderate a size as those which were then used; and it afforded greater protection from pirates than places situated immediately on the coast.