TL;DR: their concrete was different. > Roman concrete, on the other hand, is a simpler mix of quicklime made from baking and crushing limestone rocks and, most importantly, volcanic rock aggregates of various types, which were abundant in the region surrounding Rome. In contrast to the aggregates used in modern concrete, these volcanic materials used by the Romans are highly reactive and the resulting concrete remain…
The reason why multiple structures still hold today and will hold longer than most of our structures is more simple than that: they over-engineered the shit of everything that still hold today. Also for the same reasons, multiple bridges, castles, church or structures built between the low medieval era and high med/renaissance will likely last as much as roman structures did.