A small Ottoman castle on the hilltop above the old town, rebuilt by Suleiman the Magnificent in 1522 on top of an older Byzantine fortress, with seven rooms now housing the Marmaris Archaeological Museum. The collection is modest — amphorae from local shipwrecks, Hellenistic and Roman finds from the surrounding peninsula, an exhibit on the bay's history as a Mediterranean naval staging post — but enough for a slow hour.
The real reason to climb up is the view: the castle commands the harbour, the marina, and the line of pine-covered hills around the bay, and is the best photograph of Marmaris you will get. Entry is cheap, the walk up through the old town adds the only patch of historic streetscape the town has left.