Topkapi Cistern: one of the water reservoirs of ancient Byzantium, in the gardens of the Topkapı Palace.
When Byzantium, renamed Constantinople, became the capital of the Roman Empire, it soon had more inhabitants than it could supply with the water of its wells and the little river west of it. So, large cisterns were built and old reservoirs were repaired and expanded.
The Topkapı Cistern one can be found in the southwestern garden of the famous Topkapı Palace, the residence of the Ottoman sultans, about halfway in the lane between the Orta Kapı and the Bab üs-Saadet. The Topkapı Palace is built on the pagan acropolis of ancient Byzantium, and it is possible that this cistern dates back to the Classical or perhaps even the Greek age. Like the Basilica and Binbirderek Cisterns (and unlike the reservoirs of Aetius and Saint Mocius), it was covered. It was certainly in use when Constantinople flourished, and was also in use when the Ottoman sultans built their splendid palace.