...must have dominated a large part of medieval Constantinople, it's still impressive today and it's hard to believe how old it is. (Emperor Valens built it in the late 4th century AD!) The Valens Aqueduct brought fresh water into the heart of the City and piped water into many of the palaces and fountains.
This is how the Valens Aqueduct looked when we went on our research trip to Istanbul in the early spring of 2009.
But nothing beats actually seeing a place. I am not sure whether the cistern this aqueduct fed into would have resembled the Basilica Cistern, but seeing the Basilica Cistern and the Aqueduct did inspire a couple of important scenes in the Palace Brides trilogy.
*** (Permission to use the map is granted on Wikepedia. Topographical map of Constantinople during the Byzantine period. Main map source: R. Janin, Constantinople Byzantine. Developpement urbain et repertoire topographique. Road network and some other details based on Dumbarton Oaks Papers 54; data on many churches, especially unidentified ones, taken from the University of New York's The Byzantine Churches of Istanbul project. Other published maps and accounts of the city have been used for corroboration.)