The Aya Sofia is the one image that flashes before our eyes when someone mentions the city of Istanbul. It is quite simply and with considerable justification, one of the most recognized buildings in the world. It was built over the ruins of two older churches in 537 by Justinian the Great who then ruled the Byzantine Empire. It was the largest place of worship in Christiandom until the completion of St. Peter's in Rome one thousand years later. The Aya Sofia was a cathedral until the Turkish conquest of Constaninople in 1453 when it was turned into a mosque. The Moslems built four minerats at the corners of building at this time. Ataturk turned the mosque into a museum in 1935. The Aya Sofia is entered through the east entrance where you actually pass some of the remains of the earlier churches. As you pass into the exonarthex which is vaulted outer vestibule before entering the inner vestibule. Here overhead you can see some of the original mosaics from Justinians time. They are quite stunning, designed in various geometric shapes in wonderful colours. From here you pass through the Imperial Gandt and underneath the stupendous dome. The dome is remarkable for its size and the height that it soars over those that behold it. It is 56m high and 31m across. With various mosaics and calligraphic discs(the largest in the Islamic world) spread across the walls and the ceiling, the building becomes quite awe inspiring. I cannot begin to list all of the marvels of this wonderful structure without exhausting myself. However the must-sees include a visit to the first floor where you will find some of the most remarkable mosaics in Christiandom. Most of these are on the southend of the floor in the Empress Zoe Gallery. They date from the 9th and 10th century. It should be noted that the Aya Sofia is justifiably very popular, hence it is very crowded. Wisely I visited first think in the morning. Thankfully I was staying just 300m from the building. Still it became incredibly crowded in no time so be patient when you visit.