Founded as the Greek colony of Cius in antiquity, this strategic port city flourished under Bithynian rule when King Prusias I rebuilt and renamed it in the 3rd century BCE. Situated where the Sea of Marmara meets the Gulf of Izmit, its ruins today include fragmentary walls, theater foundations, and submerged harbor structures visible from coastal walking paths.
Visitors can explore the overgrown acropolis hill with scattered column drums and inscriptions, while local fishermen still use the ancient breakwaters. The site offers exceptional sunset views across the water to modern Karamürsel. Wear sturdy shoes for uneven terrain and visit in spring to avoid summer heat - the nearby Princes' Islands make a perfect combined day trip from Istanbul.