A notice explains this is called the “Roman door”. I think that is a bad translation of the Turkish word kapı, so let’s call it the Roman Gate. The notice describes how the Greek Gate between two towers a little further used to be connected to this Roman one, and how in the third century AD, in reaction to increasing threats to the city, the walls of the city were built anew in such a way the Roman Gate became one of the entrances to the city. On one side the gate has five large niches, with the gate in the middle one. A marble arch was erected over the gate to give it a more impressive appearance during or shortly after the reign of Septimus Severus. Sculptures of Athena, Artemis, Aphodite, Nemesis and Asklepion that adorned it are now in the Antalya Museum (see my gallery of that museum).
On some pictures you can see the relationship with the round Hellenistic Gate towers.