This gate appears to be in poor condition. However, when you realize that it was built in AD 69, the strength of masonry arches becomes evident. It is just one of several similar gates that were constructed.
Oddly, it is likely that this was not built as a defensive work - the Romans had so successfully brought order to their realm that attacks were unlikely. Any hostile actions were much more likely to be fought in the field that at the wall. The gate probably only marked the entrance to a chartered city.
Notice how sad the area seems. This is clearly the site of a once-grand city. At the time the photograph was taken, the area appears all but deserted, the surrounding buildings have fallen into disrepair, and the bustle that must once have characterized the area is gone.
Part of the reason for the decay may be that people forgot the need to create enclosed spaces within cities. Here, the spaces are far too large for the level of activity, and the enclosure is broken. The gate remains as a reminder of by-gone grandeur.
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