Hagia Sophia (Turkish: Ayasofya) is perhaps the best-known site in Istanbul. From the date of its dedication in 360 until 1453, it served as an Eastern Orthodox cathedral and seat of the Patriarchate of Constantinople; except between 1204 and 1261, when it was converted to a Roman Catholic cathedral under the Latin Empire. The building was a mosque from 1453 until 1931, when it was secularized. It was opened as a museum on 1 February 1935.
It is considered the epitome of Byzantine architecture - it remained the world's largest cathedral for nearly a thousand years thereafter, until Seville Cathedral was completed in 1520. Its famous in particular for its massive dome, which can be seen on the 2nd postcard.
A personal favourite - Basilica Cistern, the largest of several hundred ancient cisterns that lie beneath the city. Built in the 6th century during the reign of Byzantine Emperor Justinian, it is located some 150 meters southwest of the Hagia Sophia, and be careful cos you might miss it! It may have the capacity of 80000 cubic metres but its not conspicuous from the outside.
This postcard I received just before my trip to Istanbul, as a sort of an invitation :) I like the Galata Tower, especially in the night under those lights. Its a nine-story medieval stone tower (66.90 meters tall) in the Galata/Karaköy quarter, just to the north of the Golden Horn.
In the end what I found most fascinating about Istanbul is the skyline dotted with numerous imposing mosques...and just how tasty a street kebab can be :))