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This was built by Nasreddin Aslan Mehmed, Emir of Saltuk, in 1179 AD / 574 AH. It's rather plain outside and within, but an impressive space with room for 10,000 worshippers.
Atop the hill at the heart of the city, this was established by the Byzantines in 422 AD but what you see now is mostly Seljuk, 12th century. It includes a museum and a mosque: you can climb the 19th century Tepsi minaret / watchtower for views. The outer castle walls have disappeared.
Çifte Minareli Medrese
Built as a madrasa - a theological school - circa 1260, it takes its name from the two fluted minarets that crown the monumental façade. It's now a museum and city symbol.
19th century merchant's house, used as the German consulate 1915/16, and later as the Governor's residence. It accommodated Atatürk for 52 days in 1919. Signage is only in Turkish.