Fort Queyras (also called Château-Queyras) is a hilltop fortress in the commune of Château-Ville-Vieille, perched on a glacial knob that the Guil river squeezes around. The site has been fortified since at least the 13th century; what you see today is largely Vauban's late-17th-century reworking.
The fort is open to visitors in summer with self-guided tours of the ramparts, dungeons, and a small museum on the Vauban defence system. The view down the valley toward Italy is the real prize.