At around sixteen hundred metres, Mont-Louis is often described as the highest fortified town in France, and it feels like a frontier outpost even today. Vauban designed it in 1679 to guard the newly drawn border after the Treaty of the Pyrenees, placing it on a strategic shelf at the meeting of three valleys: the Têt dropping towards Perpignan, the Aude running north, and the wide basin of the Cerdanya opening to the west. The citadel is still used by the French army, home to a commando training centre, so parts of it are closed to visitors, but the walls and the small grid of streets inside them are free to wander.
The town's oddest treasure is its solar furnace, built in 1949, the first of its kind in the world. A field of mirrors concentrates sunlight onto a target, producing extreme temperatures, and it is possible to visit. The much larger furnace at Odeillo, near Font-Romeu, followed later and is visible from the road, its huge curved mirror reflecting the mountains.
The riding around Mont-Louis is some of the most varied in the eastern Pyrenees. Climbing up from Villefranche, the road twists through the Têt gorges in a series of hairpins and long, swooping bends, with the yellow train often keeping you company on its narrow tracks. Just above town, the Col de la Perche at 1,581 metres leads gently into the sunny Cerdanya plateau, with long open roads and views towards the peaks on the Spanish side. North, the road towards the Capcir plateau and the Aude gorges is quieter and slightly wilder, crossing pine forests and high meadows.
It can get surprisingly cold here, even in summer, and snow can arrive early in autumn and linger late into spring, so check the forecast and pack layers. Fuel and supplies are available in Font-Romeu and the surrounding villages, while Puigcerdà, just over the Spanish border eighteen kilometres away, is the largest town in the area and a lively place to stop for lunch. It is also worth knowing that the Cerdanya is one of the sunniest corners of France, which is why the solar scientists came here in the first place.