The Charles Bridge across the Vltava River in Prague, Czech Republic, was built between 1357 and 1402 AD. It's been in use ever since, apart from brief periods when weather or war damage had to be repaired, and is currently undergoing a major 20-year restoration.
Courtesy of Matt Bracken on Gab, here's a fascinating video showing how our medieval forefathers constructed the bridge. It took them 45 years, using only human and animal muscles and the most primitive of equipment. The fact that it's stood so long, and served so well since then, is pretty amazing.