Iznik Castle is the most important tourist attraction in the city, as it attracts large numbers of tourists and Turks wishing to get acquainted with this ancient building, which was used for defensive purposes in order to protect the city throughout history. The construction of the castle dates back to the third century BC and includes several doors on its different sides, some of which are main and others are subsidiary. The largest part of this castle is still standing today, which encourages tourists to wander around.