Gaziantep, the biggest city in southeastern Turkey, is one of the first settlements in Anatolia. The Gaziantep Fortress is one of the best examples of fortresses in Turkey. Research has found that it was inhabited during the Chalcolithic Age (4,000-3,300 BC). Its present form dates from the 6th Century AD. There is a mosque, a cistern and some ruins in the castle, and in the basement, caves, corridors, rooms and a water source may be seen.