Istanbul: Aya Sofya

This structure is really amazing, and Wikipedia's description is far better than mine could be:

Hagia Sophia (from the Greek: Ἁγία Σοφία, "Holy Wisdom"; Latin: Sancta Sophia orSancta Sapientia; Turkish: Aya Sofya) is a former Orthodox patriarchal basilica, later a mosque, and now a museum in Istanbul, Turkey. From the date of its dedication in 360 until 1453, it served as the cathedral of Constantinople, except between 1204 and 1261, when it was converted to a Roman Catholic cathedral under the Latin Patriarch of Constantinople of the Western Crusader established Latin Empire. The building was a mosque from 29 May 1453 until 1931, when it was secularized. It was opened as a museum on 1 February 1935.

Our pictures aren't that great because the space is so dark, but it gives it a certain mystique.