Zeyrek Mosque, formerly the Monastery of the Pantokrator, is a large mosque in Istanbul’s Zeyrek district, overlooking the Golden Horn. Comprising two former Byzantine churches and a chapel, it is the city's best-preserved example of Middle Byzantine architecture and the second-largest surviving Byzantine religious structure after Hagia Sophia.
Built between 1118 and 1136, the complex was founded by Empress Irene of Hungary and later expanded by Emperor John II Komnenos. It housed a monastery, a library, a hospital, and an imperial mausoleum. During Latin rule (1204–1261), it was used by Venetian clergy and later restored to Orthodox monks.
After the Ottoman conquest in 1453, it became a mosque and an Islamic school, named after scholar Molla Zeyrek. Over time, the complex fell into disrepair but was restored in the 21st century and reopened for prayer.
Architecturally, the mosque features Middle Byzantine elements, including cross-shaped churches, domes, and polygonal apses. Though much of its rich decoration is lost, fragments of marble, stained glass, and mosaics remain. The nearby Şeyh Süleyman Mescidi may have been part of the original monastery.
Visby Cathedral (also known as St. Mary’s Church) is the only survived medieval church in Visby. It was originally built for German merchants and inaugurated in 1225. Around the year 1350 the church was enlarged and converted into a basilica. The two-storey magazine was also added then above the nave as a warehouse for merchants.
Following the Reformation, the church was transformed into a parish church for the town of Visby. All other churches were abandoned. Shortly after the Reformation, in 1572, Gotland was made into its own Diocese, and the church designated its cathedral.
There is not much left of the original interior. The font is made of local red marble in the 13th century. The pulpit was made in Lübeck in 1684. There are 400 graves under the church floor.