Located at the top of the historical peninsula and surrounded by the city walls, İçkale is significant, with archaeological excavations dating from antiquity. This area is also surrounded by ramparts and was constructed as an interior castle. Findings unearthed in the excavations conducted in the interior fortress area reveal that this historical site in the city's acropolis was used during the Roman, Byzantine, Seljuk and Ottoman periods, starting from the third century BC. During the excavations, a bronze coin of Tryphon (160-138 BC), found at the Gate of İçkale, is believed to be among the oldest written documents.
Coins discovered in the excavations attributed to the periods of Justinus II (565-578), Constans II (641-668) and Leo IV (886-912) indicate that the settlement of the city continued during the Byzantine period and between the 6th and 10th centuries. At the site of a planned, central domed triconch (three-leaf clover) church, a three-nave basilica dating from the 6th century was found below; this was demolished by the 10th century and, rather than the church, a necropolis was constructed. The current church was built at the end of the 12th century.
Following the conquest of the city in 1221 and taking advantage of the remains of the older structures, the reconstruction of İçkale was reconsidered to establish the Seljuk Palace, the residence of the Seljuk Sultan Aladdin Keykubad I. Keykubad’s palace consisted of a complex comprising a front courtyard surrounded by two-level rooms, a ceremonial veranda where the throne was located, rooms reserved for the Sultan’s quarters, a pinnacle and a harem. The excavations indicate that the palace was richly decorated, with mosaic floors and wall frescos.
The other buildings in İçkale include the monumental fortress gate, the two-level vaulted galleries, two-storey navigational pavilions and other important structures remaining from the Seljuk Period. This area was transformed into a 'Cistern Castle' in the Ottoman era, when settlement in the upper part of the city required a need for water.