Great Mosque is the oldest mosque in Turkey which was built in Islamic style. It was built by the last Umayyad Khalifa, Mervan II, between the years 744-750. The area on which the mosque was built is 104 x 107 meters. The height of its minaret, which has still stood today, is 33.30 meters. For a long time, it was claimed that this minaret had been an observatory; however, later on it was understood that it was the minaret of the mosque. It is written that the minaret can be reached through wooden stairs with 105 steps. In the east part of the castle, excavation studies have been carried out under the presidency of Prof. Mehmet Önal since 2014. Turkish bath, water well with platform, cisterns, a courtyard with a water-tank with a fountain, a covered bazaar, shops of the market, and a way were explored during these excavations.