It’s a slight mystery to me that this hasn’t fallen down. It was constructed in 131AD as a monumental gateway with some people thinking it marked the entrance to the new part of the city, others disagreeing. But such is history…
I’d add though that a lot of buildings and structures which have been constructed in recent times have already either fallen down or been pulled down, so I have to credit the Roman civil engineers. Although I expect they had a lot of philosophers to help them.
Anyway, when the Romans left Athens it sort of got abandoned and was left to the elements. There’s some evidence that it was later used as a wall of a church, which might explain why it was protected for so long. The bottom portion of it also became buried down the centuries, which helped to ensure that it didn’t erode away at the base.
The base was uncovered following an archaeological dig, the entire arch is now visible once again. The downside of this is that the gate has been left open to the pollution of the city, and the stonework and inscriptions have suffered over the last few decades. But, nonetheless, it is remarkable that this structure has survived.