Yoros Castle stands near the northern end of the Bosphorus, close to where the strait opens toward the Black Sea. The ruins are fragmentary, but the setting makes the strategic point obvious: ships, currents, wooded hills, and the mouth of the waterway all come together here.
Come for the view and the sense of distance from central Istanbul. It works best as part of a Bosphorus day out rather than as a quick city-centre stop.