The largest church building in the world for over a thousand years, the Hagia Sophia in Istanbul is an absolute 'must-visit' place in Istanbul. In Greek its name means 'Holy Wisdom', although it is often mistakenly translated as 'Saint Sophia'. In Turkish it's pronounced Ayasofya, but people will know what you're talking about if you ask for it as the Hagia Sophia.
Its domes remind one of a mosque, but the wonderful reality is that it pre-dates the major mosques, and its architecture inspired them. The domes have long been studied for their architectural importance, and lend a sweeping magnificence to the interior.
The building was completed in 537 and was used as an Eastern Orthodox Cathedral and seat of the Patriarchate of Constantinople, apart from a brief period when it was converted to a Roman Catholic cathedral (1204-1261) under the Latin empire. In 1453, with the fall of Constantinople and the Holy Roman Empire, it was taken over by the Ottomans.
From 29th May 1453 - 1931 it was a mosque. Representations of the human form are not permitted in a mosque, so the frescoes were all covered with a whitewash. This has since been removed and a many of the images restored. Arabic inscriptions around the building remind one of its different uses.
On 1st February 1935 the secularized building was reopened as a museum, and it has remained open to the general public.
Another Muslim change was the inclusion of a mihrab and mimbar at one end of the building. The mihrab is a semi-circular niche showing the direction of Mecca, while the mimbar is a pulpit from which the imam can preach at services. These have been left intact and are beautiful to behold in their own rights.
It was put up in very little time, largely because the materials were borrowed from other buildings, including ancient monuments. This includes columns from the temple of Artemis at Ephesus. One particularly impressive example is Justinian's use of eight Corinthian columns from the Temple of Jupiter at Baalbek, in the Bekaa Valley, Lebanon - a long way to drag the heavy stone.
At the end of the narthex there's an entrance which takes you up to the higher level. This isn't advisable if you are at all unsteady for your feet as it is an uneven cobbled slope rather than stairs. It is worth going up, as at the top there are exhibits about the objects pertaining to this amazing building, but also exceptional views. It is not, however, possible to walk all the way around the gallery. Up here is also the empress's loge, the enclosure in which she could sit in order to observe the action below.
Fabulous mosaics adorn the walls and ceilings of this remarkable building. On your way in you can observe, for example, the emperor bowing down before Christ, designated as pantokrator, or ruler of all.
In the apse, above where the altar would have stood is a Virgin and Child, the first post-iconoclastic mosaic in the building (dated 867). This may well be a reconstruction of the 6th century original mosaic, destroyed in the Byzantine attacks on iconoclasm.
Other highlights include an intricate marble door, lustration urns, and a wishing column. It's hard not to be overwhelmed by the gilding in here, which offers a warm glow to the whole building.
Outside the front lie the foundations of a previous church on the site - this has been hallowed land for millennia, and it sends a shiver down your spine to contemplate all the devotion which has happened here. Whether or not you are religious, this is a stunning space which really should feature on your itinerary.
It's located in the old section of the town. You can get the tram there from various other parts of Istanbul. More travel information is available on the website. It's right next to the 'Blue' Mosque, close to the Grand Bazaar, Spice Bazaar and Topkapi Palace, so it would be really easy to spend a day or more just in this area of town, exploring both these major sites and some smaller Roman remains.