Roman harbours

A Roman harbour was often more than a quay. Where the coast offered no natural shelter, the Romans dug one: the sea harbour that Claudius began at Portus in AD 42 was an excavated basin with moles on either side of its entrance and an island carrying a lighthouse, linked to the Tiber by a canal (the operator; en.wikipedia). Silting and storms led Trajan to build a second, hexagonal basin further inland (the operator).

A harbour is harder to read than a temple or an amphitheatre, because most of it was water. At Portus the hexagon is still filled with water, beside the remains of warehouses, and the town that grew up around the harbour was walled at the end of the 5th century and became the seat of a diocese before it emptied (the operator; it.wikipedia).

Portus was built because the river port of Ostia could no longer handle Rome’s traffic (the operator), so the two sites explain each other.