Hagia Sophia: Initially built as Church in 500 AD. Later on converted to mosque and then to museum. Interesting and beautiful historical monument representing the beginning of middle ages. Famous for its massive dome, byzantine arts and marble structures. Its a true representation of Istanbul where east meets west.
Blue Mosque: Built in 1600 AD.
Life along Bosphorus river:
Basilica Cistern: Another historical monument from 6th century built under the old city of Istanbul.
Chora Church: Pilgrimage for Catholics. Built around 500AD, considered as birth place of Chora movement (Byzantine art).