Hagia Sophia is a very famous historical monument in Istanbul. Hagia Sophia is a mosque, a museum and a church. This building was built by the Byzantine Emperor Justinyanus, the famous architects of the time. It is said that its history dates back to the 5th century. Hagia Sophia consists of many parts. These structures were built by different Byzantine emperors in different periods. With the conquest of Istanbul by the Turks, the mosque became a mosque, too.