They were built during the period of emperor Nicomedes I (278 – 250 BC); Kocaeli’s exterior walls were built while Bithynia was the centrum of the kingdom. The walls from the west to east measure 75-100 m length, and from north to south 50 m largeness. The north and south-west walls are undergird with towers. Izmit walls have a vital importance regarding their constructing techniques and the brickwork used in the Roman era. On the Roman walls, from place to place, can be seen pieces from the Hellenistic period. The last renovations of the tower were made during the Ottoman period.