It might be hard to see in this picture but the fortress runs up the slope in the center of the picture. The complex is really four forts linked by a covered stairway of almost 4,000 steps. There is a museum as well as a cafe at the bottom.
La Passeggiata Reale is a visit around the San Carlo fort and a trip up the covered stairs. It starts in the morning at nine and returns at 4 in the afternoon.
You can take a walk just inside the walls. It takes three hours, and you can start at 10 in the morning or three in the afternoon.
The above times might change, see this fortezza di Fenestrelle page.

