While a precise chronology for the post-Honorian additions to the Porta Appia is difficult, a few remarks may be hazarded.

As others have said, the foundations of the Honorian towers subsided rather dramatically at an early date, destabilizing them dangerously and opening large fissures in their exteriors, some of which were extensively repaired in the 1920s. It has thus been suggested that the additions to the towers were meant to buttress them, tempting some to connect their construction with damage sustained in the earthquake of 443. This hypothesis is not without its problems, however. To begin with, its proponents assert that the new top chambers were added after the “buttresses”; yet if the towers were unstable enough already to require buttressing, it is hardly probable that they would subsequently have been singled out, as very few towers anywhere along the Wall were, for further heightening. More importantly, similar square shells were added around Honorian towers at three other gates, the Portae Flaminia, Tiburtina, and Praenestina-Labicana, none of which seems to have been affected by serious problems with subsidence, earthquake-related or otherwise.

The fact remains that both the square casings and the top chambers do appear to have been added after the gate had suffered significant damage, consonant with what an earthquake might be expected to provoke. The year 443 thus remains a possible terminus post quem; but as it seems unlikely that reconstruction on such a grandiose scale was undertaken during the turbulent years between the sack of Rome in 410 and the end of the fifth century, I am inclined to move the terminus post to the period of Theoderic, the first ruler after Honorius to whom significant work on the Wall can be credited, during whose reign another powerful earthquake shook the city, in 502.