Peyrepertuse

The fortress was built in the 11th century in a strategically important place, on the border of France and Spain, on a rock 800 meters high. The two castles are connected by a large stone staircase. In 1659, the border between France and Spain was moved, and the castle was abandoned as no longer needed.

It is now open to visitors. Information here.

I really liked the ruins; they are large and picturesque.