Aya Sofya (known also as the Church of the Divine Wisdom) was regarded as the greatest church in Christendom up until the fall of Constantinople, when it was put back into service as a mosque. The edifice is crammed with fine mosaics and topped by a magnificent dome.
Emperor Justinian (r. 527-65) completed the church in 537, as another effort to restore the greatness of the Roman Empire.
Examining the interior of the church is more a metaphysical than a physical experience. Visitors entering through the main entrance, via the low original steps, experience both a gradual sense of being drawn upwards and a sense of gloomy darkness being dispelled by the inner light of 30 million gold tesserae (mosaic tiles).
The dome is supported by 40 massive ribs constructed of special hollow bricks made in Rhodes from a unique light, porous clay, resting on huge pillars concealed in the interior walls.
It was through the Imperial Door that Mehmet the Conqueror came in 1453 to take possession for Islam of the greatest religious edifice in the world. Before he entered, historians tell us, he sprinkled earth on his head in a gesture of humility.
Aya Sofya remained a mosque until 1935, when Atatürk proclaimed it a museum. It must be seen to be believed.
Basilica Cistern, with its 336 columns and a high, vaulted ceiling, is an incredible Byzantine relic that was primarily used to store the city's water. Children in particular are fascinated by its eerie darkness and the ghostly echo of drops falling from the vaulted ceilings.
Built in AD 532, it is the largest surviving Byzantine cistern in İstanbul. It was constructed by Justinian, who was incapable of thinking in small terms. Columns, capitals and plinths from ruined buildings were used in its construction. Two columns in the northwestern corner are supported by two blocks carved into Medusa heads.
Information: LonelyPlanet.