The most photographed sight in Segovia is the magnificent aqueduct. Over 800m of granite, supported by 166 arches and 120 pillars and at its highest point towering some 30m above the Plaza de Azoguejo, it stands up without a drop of mortar or cement. No one knows exactly when it was built, but it was probably around the end of the first century AD under either Emperor Domitian or Trajan. It no longer carries water from the Río Acebeda to the city, and in recent years traffic vibration and pollution have been threatening to undermine the entire structure, but the completion of a meticulous restoration programme should ensure it remains standing for some time to come. If you climb the stairs beside the aqueduct you can get a view looking down over it from a surviving fragment of the city walls.