Any Roman city of importance will be served by an aqueduct.  Segovia was obviously one such.  That its aqueduct is still standing, 1500 years after the collapse of the empire that built it, is a marvel.

As it turns out, this aqueduct supplied water to Segovia until about 50 years ago, guiding mountain streams into the town's water works ceaselessly for almost two millenia.

We moderns look at this ancient technology and we envy the engineers whose names are lost in the mists of time.  But their handiwork is still here.

As tall as a six-storey building, the typical gradient of a Roman aqueduct was on the order of 34cm (13") per kilometer (3280ft).  It doesn't sound like much, but it could deliver 20,000m³ per day and that was enough to power all the fountains in a town the size of Segovia.

The Romans had a recipe for mortar that (we think) was better than what we use today.  Even so, they didn't mortar the joints in aqueducts, possibly because the mortar would contaminate the water (?)  That means you are looking here at a construction that is simply stones laid upon stones with nothing but gravity to keep them from moving.

These stones have been in this arrangement since this aqueduct was first constructed 2,000 years ago — give or take a few hundred years.