The Zeyrek Mosque in Istanbul presents a fascinating story through its architecture and history. Originally constructed as the Monastery of Pantokrator in 1118, this site is actually a combination of three distinct buildings. These include the main church, a second public church added around 1136, and a chapel that functioned as a royal mausoleum.
During the Ottoman period, the complex was converted into a mosque and a medrese, an Islamic school. Remarkably, this transformation required very little structural change, allowing the site to retain its original design features. This makes the Zeyrek Mosque one of the best-preserved examples of late Byzantine architecture in Istanbul.
Today, the Zeyrek Mosque continues to function as a place of worship. Its beautiful facades and domes attract photographers, especially in the daylight or at sunset. The surrounding neighborhood, however, is generally quiet after dark and is not recommended for visitors at night.
Source - https://en.wikipedia.org/wiki/Zeyrek_Mosque