Eskihisar Castle, which is thought to have been built in the 1200s and became one of Byzantium’s most important defense lines, has stood for 9 centuries. The castle is shown as one of the most important castles protecting the sea routes to the capital city Istanbul for centuries, and is located in the Gebze district of Kocaeli, where the most important trade routes of the period passed. Unlike the defensive castles built at the time, Eskihisar Castle, which also holds an interior castle, is located 35 meters above sea level.