Hagia Sophia (Church of the Holy Wisdom of God) is considered as the epitome of Byzantine architecture, famous in particular for its massive dome. Constructed as a church between 532 and 537 AD on the orders of the Byzantine Emperor Justinian, it remained as the largest cathedral ever built in the world for nearly a thousand years.
Having been the religious focal point of the Eastern Orthodox Church for nearly 1000 years, it was converted into a mosque in 1453, the day after Constantinople was conquered by the Ottoman Turks led by Sultan Mehmed II. The bells, altar, iconostasis, and sacrificial vessels were removed, and many of the mosaics were eventually plastered over (note). The Islamic features - such as the mihrab, the minbar, and the four minarets outside - were added over the course of its history under the Ottomans. Perhaps the most well known restoration of the Hagia Sophia was completed between 1847-49 during the rule of Abdülmecid II, who invited Swiss architects Gaspare and Guiseppe Fossati to renovate the building. In addition to consolidating the dome and vaults and straightening columns, the two architects brothers revised the decoration of the exterior and the interior. The discovery of the figural mosaics after the secularization of Hagia Sophia in 1935, when it was converted into a museum, was guided by the descriptions of the Fossati brothers who uncovered them a century earlier for cleaning and recording.
Note: Following the building's conversion into a mosque in 1453, many of its mosaics were destroyed or covered with plaster, due to Islam's ban on representational imagery. This process was not completed at once, and reports exist from the 17th century in which travellers note that they could still see Christian images in the former church.