One of the oldest buildings in the world, Hagia Sophia Museum was built as a church in 537. The building, which was converted into a mosque after the conquest of Istanbul in 1453, was converted into a museum in 1935.
Visited by millions of people throughout the year, Hagia Sophia is also valuable in terms of hosting the world's best architectural features. Indeed, when you enter, the Hagia Sophia Museum, which you admire against all the details of its architecture, is open to visitors seven days a week.
Although the entries are a bit expensive, you can get a free ride by getting a Museumcard.