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Photo: Dosseman, CC BY-SA 4.0 via Wikimedia Commons
Byzantine monastery complex built in 463 AD. In the 15th century it served as İmrahor Mosque, but was wrecked by fires and earthquakes. A ruin it remains, and plans to restore it as a mosque have come to nothing. You'll probably find it locked.
The Monastery of Stoudios, more fully Monastery of Saint John the Forerunner "at Stoudios", often shortened to Stoudios, Studion or Stoudion, was a Greek Orthodox monastery in Constantinople, the capital of the Byzantine Empire. The residents of the monastery were referred to as Stoudites or Studites. Although the monastery has been derelict for half a millennium, the laws and customs of the Stoudion were taken as models by the monks of Mount Athos and of many other monasteries of the Orthodox world; even today they have influence.
© OpenStreetMap contributors
The Golden Gate was the ceremonial entrance through the Theodosian city walls. After the Ottomans captured the city in 1453, Mehmed II needed a stronghold for his treasures and documents. The gate was therefore bricked up and the walls reinforced into a fortress, with valuables stored in its seven towers, hence yedi kule. In the following century the treasury moved to Topkapi Palace and the fort became a prison for high-level detainees - Osman II was strangled here in 1622. It remained in use until 1837, whereupon the Golden Gate was re-opened.
Sümbül Efendi Mosque
Built in the early 5th century as a monastery dedicated to Saint Andrew the Apostle, in 766 it was the burial place of Saint Andrew of Crete and was later re-dedicated to him. It was rebuilt in the late 9th century and again in the 13th, then around 1490 converted into a mosque. From the 16th century it was occupied by the Dervishes, when the legend arose that a chain hung to a cypress tree in the courtyard was a truth diviner. The chain was swung between rival witnesses and the one it hit was telling the truth. The cypress stump is still standing.
This is a circular indoor space enclosed by a 3D depiction of the Ottomans breaching the Walls of Constantinople on 29 May 1453, with sound effects. It's over-priced, unhistorical and tourist-trappy.
The original mosque was built between 1463-1470 by the Greek architect Atik Sinan, by order of Sultan Mehmet II the Conqueror, on the site of the former Church of the Holy Apostles, which had served as Byzantine Imperial burial place for a thousand years. The grand complex with eight medreses, a library, hospital, hospice, caravanserai, market, hamam, primary school and public soup kitchen was smashed again and again by earthquakes, and the present building is from 1771 to a different design. The interior is lavish, and outside is the ornate tomb of Mehmet II and his wife Gülbahar Hatun.
Imposing mosque built 1500/05. The other parts of the religious complex have found secular use.
This is a triple structure: the church of the Monastery of Pantokrator built 1118, then a public church circa 1136 plus a chapel that served as a royal mausoleum. It became a mosque and medresse in Ottoman times, but without massive alteration so it's a prime example of middle-Byzantine architecture. It's nowadays again a mosque, photogenic by day or dusk but the area should be avoided at night.