This is the iconic monument of Istanbul. There are other famous and breathtaking monuments in this city, but this is the one that most everyone has likely seen or heard of. It was completed in 537AD by the Emperor Justinian, and for nearly 1,000 years reigned as Christianity's greatest church. When Mehmet took the city in the mid 15th century, he declared the Hagia Sophia to be a mosque, and it operated as such until becoming a museum when Ataturk took over. Go here. Don't question, just go.