Once capital of the Byzantine Empire, Istanbul (then Constantinople) is now the most populous city as well as its cultural and financial hub in Turkey and the 6th largest urban city in the world! I have always been amazed by its beauty and would love to visit one day. Here are three of the top places to visit in the old city. Hagia Sophia - Commissioned by the great Byzantine emperor Justinian, consecrated as a church in 537, converted to a mosque by Mehmet the Conqueror in 1453 and declared a museum by Atatürk in 1935 - this is the most famous monument in Istanbul. Blue Mosque - İstanbul's most photogenic building was the grand project of Sultan Ahmet I (r 1603–17), whose tomb is located on the north side of the site facing Sultanahmet Park. Note that it is closed to tourists during the 5 daily prayer times & Fri sermon. Grand Bazaar - Starting as a small warehouse built by order of Mehmet the Conqueror in 1461, it grew to cover a vast area as laneways between the warehouse and neighboring shops were roofed!