Istanbul tells the story of transition from the ancient to the modern world. It has been a bridge between continents throughout different civilizations and kept this heritage alive to this day. The city also allowed multiple cultures and religions to mingle, trade, and thrive. In fact, the Roman Empire did not collapse as is often reported in the West. Rather it moved to Constantinople. Architectural details around the city tell us much about this transition.

Basilica Cistern or Yerebatan Sarnıcı
While Constantinople is famously surrounded by water, the triangular peninsula on which the city rests lacked sufficient fresh water sources. An extensive system of aqueducts built in late antiquity helped transport water from the Belgrade Forest and other sources in Thrace up to 100 km away to the growing population of Constantinople. This system also included a large number of cisterns that stored waters supplied by aqueducts. The underground water cisterns became the major water reservoirs of Constantinople. They were built during the reign of Emperor Constantine in the 4th century and are some of the largest Byzantine structures in modern Istanbul. Major cisterns were usually placed on or near the top of hills to supply water to various buildings such as public baths, palaces, monasteries, and churches. These cisterns were especially important for the security of the city, particularly during sieges. 
There were more than 80 underground cisterns in Istanbul, but Yerebatan Sarnıcı is the largest and the best excavated.  It is the size of a cathedral and can hold up to 80,000 cubic meters (or 17.5 million gallons) of water.  Many of its 336 columns were salvaged from ruined temples and feature finely carved capitals. Famously, two giant, snake-covered Medusa heads also lie ominously in the cistern, threatening to turn any viewer into stone. Several competing theories explain why one of the Medusa heads is sideways at the base of a column and the other is completely upside-down. Perhaps they scared away intruders from contaminating these precious waters. The heads may have been removed from an ancient building called the Forum of Constantine, where similar ones have been found. The upside-down head could show that Byzantine builders perceived Roman relics as only reusable rubble. But other historians point to the early Christian practice of putting pagan statues upside-down to make a bold statement about their faith.  
The cisterns have become iconic in modern entertainment, ranging from James Bond to Marvel movies, to the bestselling novels by Dan Brown and even the popular Assassin’s Creed video games. 
These cisterns even supplied Topkapı Palace, which was not only the residence of the Ottoman sultans, but also the administrative and educational center of the state. Initially constructed in 1460 by the conqueror of Constantinople Sultan Mehmed the Second, the palace’s design played an important role in Ottoman governmental philosophy and in the relations between the palace and its subjects. Visitors would pass through a large park to cleanse their palate from the crowded Istanbul streets, and then be fed with a kitchen that could create 5,000 meals per sitting. Diplomats would pass through various protocols prior to their proposal being entertained in the sultan’s audience hall. The harem and sultan’s residence were also in the inner sanctum of the Topkapı Palace. People close to the sultan were trained to manage important representatives from around the world. Important relics from Islam were also preserved in the most sacred places since the sultan was also the caliph after the Egypt campaign. Today, the Basilica Cistern is one of the most popular places among tourists visiting Istanbul.

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