Yoros Castle

The ruins of an old Byzantine castle is located on a hill overlooking a small fishing village called Anadolu Kavağı, just outside Istambul (on the Asia side). The village itself is easily accessible by the Istanbul Long Bosphorus Tour; and it's a short, but steep walk up the hill from the docks.

The castle provides amazing views of both  the village and the confluence of the the Bosphorus straight with the Black Sea. The castle is itself closed to visitors (there are apparently architectural dogs underway), but access is free and worth it for the views alone. There are also two restaurants up there, with great fresh seafood, at awesome prices.