Trajan's Bridge

Apollodorus of Damascus built it across the Danube in 103–105 AD, to carry Trajan's army into Dacia, and for roughly a thousand years afterwards it was the longest arch bridge ever built.

What was built: masonry piers founded in the riverbed — around twenty of them — carrying a timber superstructure, with a total length of something over a kilometre. That combination explains both the achievement and why so little survives. Building stone foundations in the bed of the Danube in two campaigning seasons, without modern pumping, is the part that impressed Roman writers and still impresses engineers. It is depicted in relief on Trajan's Column in Rome, which is the primary evidence for what the roadway looked like.

Why it came down is genuinely disputed. The superstructure was gone within a century or so, and the ancient sources split between Hadrian, said to have had the timber removed so the bridge could not be used against the Empire from the far bank, and Aurelian, later, around the Roman withdrawal from Dacia in the 270s. Both make strategic sense.

What you can see: one pier on the Romanian bank, in the open and free to walk up to, with its twin visible across the water in Serbia. The piers in the river have been going steadily for a century — some removed because they obstructed navigation, others lost to the current and the raised water level after the Iron Gates dams.

So come expecting two piers two kilometres apart in two countries, not a bridge. The Iron Gates Region Museum next door is what makes it legible, and the full guide has the day.