Eskihisar Castle rises above the old harbour village and the ferry traffic crossing the Gulf of İzmit. The Byzantines are thought to have built it in the 12th century to watch the southern shore of the gulf and the port below.
The restored walls are the main draw rather than an elaborate interior. Come for the position: sea air, views over the water, and a quick sense of why this small hill mattered before modern highways and industry changed Gebze.