What are some fascinating examples of ancient or medieval technology?

Production of gunpowder, in particular of saltpeter.

Ancient Google maps aka compass:D

Can we consider SAP ancient/medieval technology?

A wheel and a compass. The first is basic, but at the same time fascinating, it shows the way our mind works, we can build complex (yeah, complex wheel, I know) systems to make our life easier. Instead of carrying things in our arms we learned to create carts and more intricate mechanisms. A compass is still a thing I cannot believe someone came up with. It's literally a pocket navigating system that can lead you to the place you need (of course if you have the required knowledge and skills), it requires no energy source or network. Again, simple, but fascinating.