St. Sophia (hagia Sophia) and Baslica Cistern of Istanbul

The most important 2 landmarks of Istanbul are St. Sophia and Yerebatan (Baslica) Cisterns. They were both built by the Byzantines and two of the few historic sites in the world that has been preserved in perfect condition until now. They are both located at the "old city" and the distance between them is only a few minutes.

Once a church, then became a Mosque and now it is a Museum. The Hagia Sophia is over 1600 years old and it is still one of the most incredible and significant buildings in the world. It has a 31.24 meters diameter dome with no apparent supports to it. Hagia Sophia, right now is arguably considered to be the 4th biggest cathedral in the world but the reality still holds true that it had been built 1,000 years before any of the first three. Still, it is not possible to classify cathedrals in size because there are many different variables like the size of the diameter of the dome or how tall it stands. For the first 1000 years after it was built, it has continuously been considered as the greatest church in Christendom.

The pendentives that hold the dome are triangular shapes that let the circular base of a dome to stand weightless on a square shaped construction . The weight of the dome is supported by these four points of piers and passed to ground. This was tried to be accomplished for a long time by the Romans but it was the Byzantines that managed to build it for the first time at Hagia Sophia. Later, the technique was used at the construction of many other European churches during the 2nd millennium. This was an idea of two geniuses of their time, Anthemios of Tralles, and Isidoros of Miletos. When Byzantine Emperor Justinian stepped into this church for the first time, he proclaimed "Solomon! I have outdone you!" After Istanbul was conquered by the Turks in 1453, it served as a mosque for another 500 years. The minarets were added to the structure after it was converted into a mosque.
By the order of Ataturk, the Turkish government changed the status of this Istanbul landmark into a museum in 1935. Some of the Byzantine mosaics still survive in the museum. Some alterations were made when it was converted to a mosque to facilitate worship facing Mecca of Arabia. Hagia Sophia is a unique architectural monument. It has a total height of 55.6 meters and its dome has a diameter of 31.24 meters. Today, Hagia Sophia is a cultural monument protected by UNESCO.

Yerebatan Cistern is the old Roman built water reservoir of Constantinople. The 336 columns of the reservoir were built by Emperor Justanian as a water support to the city. The columns were collected from old Greek monuments; two of which to the northwest have upside down Medusa heads on their bottom. While there is water dripping from the ceiling and fishes are swimming in the eternal darkness you can hear the echoes of every drip in the cistern. The Cistern is as impressive as it sounds and it is definitely one of the most important places to see while you are in Istanbul.