The Pantheon (all gods temple) in Rome, Italy. Completed in 125 C.E., it’s still used for its primary purpose 1,900 years later, and it’s still in good repair and hasn’t needed any additional buttressing. It has a well designed dome roof that is both light and strong, and it’s made of concrete which gets stronger over time.
The Cornalvo Dam in Extremadura, Spain. Also a piece of Roman work, it was constructed sometime before 200 C.E. It’s made entirely of earth covered in stone where it meets the water, and is held in place entirely by gravity. It’s still in use.
The Iron Bridge, in Ironbridge, England, which is named after the bridge. It’s the first iron bridge ever constructed. It’s been used to cross the River Severn since 1781 and is in excellent repair and daily use as a foot bridge.