"The urban child, raised on concrete and under poisonous skies, is not conditioned for the magical experiences of the natural scene." Ansel Adams, 1972
The Haghia Sophia is the most important church of Byzanthium. A 1400 years old, it impressing well preserved. In 15th century it was turned into a mosque and 4 minarets where added to it.
The dome is the most impressive of the structure. If you stand in under it, you really feel tiny...
Tip: go early to avoid the touristmass!