The Cistern of Philoxenos or Binbirdirek Cistern, is a man-made subterranean reservoir in Istanbul, situated between the Forum of Constantine and the Hippodrome of Constantinople in the Sultanahmet district. It has been restored and is now visited as a tourist attraction. The entrance is located at İmran Öktem Sokak 4. Binbirdirek Cistern is the second largest cistern in Istanbul after the Basilica Cistern.
The cistern was restored by Justinian I in the 6th century, after the Palace of Lausus was completely destroyed in a fire in 475. After the conquest of the city by the Ottomans in 1453, the cistern fell into disuse, and was forgotten until rediscovered during the construction of Fazli Pasha's palace on the same site in the 17th century.
The reservoir has a surface area of 3640 m2, storing 40,000 m3 of water. The cistern is composed of a large hypostyle chamber supported by vaults. The 224 columns, each 14 to 15 meters tall, are made of marble from nearby Marmara Island. Each column is a superposition of two columns, joined by a marble ring. The floor of the cistern was later reinforced, so that only the upper column and a short sleeve of the lower column are thus visible. The original height can be seen in an excavated pond with four columns in the middle of the cistern. Most of the columns, and also the caps, are engraved with a Greek mason's mark.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.