Fethiye Castle is a scatter of rough ruins above the town, usually folded into the same uphill walk as the Lycian rock tombs. The site has had several lives, from the ancient city of Telmessos to Byzantine and medieval defences, though what remains today is fragmentary. The reason to climb is as much the view as the masonry: Fethiye's harbour, islands, and amphitheatre of hills open below you.
There is little interpretation on the ground, and the paths can be uneven, so treat it as a viewpoint with history rather than a formal monument. Go early or near sunset, when the slope is cooler and the light over the bay is better. The castle pairs naturally with the Amyntas tombs and the old town below.