Nogent-le-Rotrou is a small town in the Perche, west of Chartres and east of Le Mans. It was once the capital of the Counts of Perche, and the old power still shows in the castle that sits above the Huisne valley.
The reason to stop is Chateau Saint-Jean, a blunt medieval fortress with one of the oldest stone keeps of its kind in France. Its museum explains the Perche and the families who controlled this border country between Normandy, Maine, and the royal lands around Paris.
The rest of Nogent is quieter: Gothic churches, the Sully tomb, river walks, and a town centre that feels more local than polished. It is not a major Loire stop, but it makes a worthwhile half-day if you are crossing the Perche or travelling between Chartres and Le Mans.