"From four arched vaults linked to one another in rectangular formation, as though from the navel, four pairs of colonnades extend to each point of the compass - as in the statue of the four-handed Apollo. Of these, three of the colonnades reach the circumference of the wall, while the fourth is shorter. Yet the shorter it is, the more beautiful it is, for it, as it were, goes to meet the palace quarter which starts nearby, and serves as a gateway to it."
The "arched vaults" is a structure known as the Tetrapylon of the Elephants. It was basically a four-way triumphal arch placed at a street junction. While not the most common of arch types in the Roman Empire, other examples are known, a particularly well-preserved one (see photo below) being at Beroea (Aleppo).
The Tetrapylon of the Elephants has also come down through history in fame as being the site at which Julian the Apostate pinned up his satirical chiding of the Antiochenes called the Misopogon (or beard-hater). This structure may take its name from having a sculpture on elephants pulling a quadriga upon it or at least a carving thereof on side panels. This elephant quadriga motif can be found on some Roman coins.