The Temple of Portunus was a product of the Republican Period of Roman architecture.  Unlike the Greeks, who favored remote settings for their holy places (e.g. the Oracle of Delphi), the Romans favored urban areas.  In a city setting many people could attend temples without having to make long (sometimes perilous) trips to their locations.  Thus, with Roman and Etruscan temple building we see an urbanization of religion (at least western religion) which is a trend that continues in the west even today.

When we look at the temple itself the Greek influence is readily apparent.  For example, we see use of the Ionic order in the temple’s columns.  In fact it could be argued that the very presence and prominence of the columns themselves is rather Greek.  Consider the purpose of the columns which wrap around the temple (those not on the porch).  These are engaged columns, which essentially means that they have no purpose other than the aesthetic.  They are carved into the wall.  They are not load-bearing, they offer no structural support, they merely serve to capture the Greek aesthetic of the columns surrounding the interior of the temple.

And yet there are some important differences between Roman and Greek temples.  The most important of all is the entrance.  Greek temples featured a stereobate, a staircase which wrapped about the entirety of the temple.  The Romans (and Etruscans) dispensed with this.  Instead of a stereobate, their temples featured a single staircase at the front of the building.  Additionally, they dispensed with the opithodomos.  Essentially, as seen below, a basic Roman temple, such as Portunus, looks like a simplified version of your average Greek temple.

Greek Temple

Roman Temple