Segovia Aqueduct
Built in the second half of the 1st© this is one of the two best surviving examples of Roman aqueducts. The other is the Pont du Gard in France, which doubles as a bridge. This one carries water from a river 15km to Segovia, and ends at the castle well. At its highest it is 28.5m and 5m wide at the top. In the city there are 167 arches. I could wax lyrical, but apart from Mike P I guess I'd lose your interest.