I will introduce you to the oldest and most magnificent structure of Istanbul.
It is a basilica planned patriarchal cathedral built by the Byzantine Emperor Justinian I in the old city center of Istanbul between 532-537, and was converted into a mosque by Fatih Sultan Mehmed after the conquest of Istanbul by the Ottomans in 1453.
After the conquest of Istanbul by the Ottoman Turks in 1453, the Hagia Sophia Church was converted into a mosque and minarets were added as a symbol of the conquest.
You can watch the Hagia Sophia Sightseeing Tour video I shot for my Youtube channel from the link and see the details.
Hagia Sophia Virtual Tour