You must visit the Hagia Sophia Museum in Istanbul

The Hagia Sophia in Istanbul is a very special historical building and museum that attracts millions of tourists every year. It was built as a church in 537 under the Byzantine Emperor Justinian I. For over 900 years the Hagia Sophia was the most important building of the Eastern Christianity. The Ottoman Turks converted it into a mosque after the Conquest in 1453. It became a museum 1935.