I quote from the only decent source I found, ArchNet (fine site, by the way): "The mosque dedicated to the Sufi philosopher Şeyh Kutbuddin is located near the Yenisehir Gate of Iznik, across the street from the Nilüfer Hatun Soup Kitchen and the Green Mosque. Its exact date of construction is not known. The tomb adjoining the mosque is documented to belong to Mehmed Muhyiddin, known as Seyh or Mevlana Kutbeddin, son of the prominent Seyh Kutbeddin-i Izniki. Based on two different dates of death for the Sheikh, the tomb is dated early or late 15th century, built either by Çandarli Ibrahim Pasa (grand-vizier under Murad II and Mehmed II) or by the late Çandarli Ibrahim Pasa (grand-vizier under Bayezid I). The mosque and minaret are assumed to be contemporaneous or to be built at a later date by the grand-vizier Çandarli Halil Pasa, son of the early Çandarli Ibrahim.
Built as a single-unit domed sanctuary with a two-bay portico, only the western wall remains of the mosque, which was damaged during the Turco-Greek war of 1920-22. This wall is continuous with the eastern wall of the restored tomb. The portico arches has collapsed after the 40s and the end arch of the portico and its Byzantine column and capital, which were attached to the tomb's wall until recently, are gone. The tomb, entered east, consists of a room half the size of the mosque that is crowned by a conical dome on a dodecagonal drum. It contains two sarcophagi without inscriptions. A brick minaret with a single saw-tooth balcony rises from the northwestern corner of the tomb. It remains without spire or tiles. The construction of the mosque and the tomb is brick and stone."
ArchNet has a picture of what is a ruin. The current restored mosque was great fun to me, as there was a group of kids with their father / imam. They wanted to have their picture taken, then demanded I’d hand my camera over so they’d do it themselves. The result shows taking pictures is kids’ play nowadays. I promised they’d see the result, so here are some unusual pictures.