Château de Châteaudun

You're standing at the Château de Châteaudun, perched on a limestone outcrop high above the river Loir. Start with that big, round tower. That's the keep, built around 1170 for Thibaut V, Count of Blois. Its walls rise thirty-one metres, and with the roof it reaches forty-two. The entrance used to be ten metres off the ground. So to get in, you needed a ladder or a stair. Very useful when enemies came knocking.