Constantine founded the new capital of the Roman Empire in a promontory jutting into the Bosphorus because it could be defended so easily. Draw an iron chain across the Bosphorus and you deny ships access by sea. Build a defensive wall at the western end of the promontory, and you deny access by land. This was impeccable military logic, and it was a thousand years before an enemy could enter the city.
The lack of drinking water did not trouble Roman engineers, who were experts at building networks of the gently sloping aqueducts which would bring water to a city through a system powered only by gravity. While rebuilding Constantine’s city two centuries later, Justinian built huge underground reservoirs to store water even if an enemy could break the aqueducts. The immense cistern (it can store 800 million liters of water) had a water filtration system, and remained in use until late Ottoman times.
We walked across from Sultanahmet square, stood in a short queue, and then walked down the damp and slippery steps to the bottom of the cistern. Fortunately there is anti-skid bump tiling, and railings on the steps. In the past you could take boats through the cistern, but that more romantic custom stopped in 1985. The two Medusa head columns have become minor wishing wells, as you can see from these photos. The vaulted roofs, the dim lights, the occasional sculpted “hen’s eye” columns, all make this piece of Roman engineering a very photogenic place. So it is not a surprise that several movies have been shot here.