The Seine does something unusual below Mantes: it stops running in straight lines and starts to loop, and at La Roche-Guyon it presses right up against a wall of white chalk. The cliff is the whole point of the stop. It rises behind the village in pale, stained tiers, with houses and garages tucked into its foot, and some of those rooms are caves rather than rooms, cut straight back into the rock. At about thirty metres above sea level you are on the river's own floor, and the cliff makes it feel like the bottom of a bowl.
The riding here is gentle and rewards a slow pace. The riverside road follows the bend with long, easy curves, plane trees and poplars on one side, rock on the other, and the odd barge sliding past at walking speed. Traffic thickens on fine Sundays because Parisians come out for exactly this view, so an early start gives you the village almost to yourself. The surface is decent chalk-country tarmac, but after rain the cliff drips onto the lane in places, so respect the shaded sections.
The château is what turns this from a pretty bend into a story. A medieval keep climbs the cliff behind the village, reached by a stair cut into the rock, while the later house sits below it, partly built into the cave dwellings. In 1944 the German commander Erwin Rommel made his headquarters in the château during the build-up to the Normandy landings, which gives the quiet streets an odd weight. The village is listed among the most beautiful in France, which is fair, and also small enough that you will see all of it in half an hour.
Gasny and Freneuse are a few kilometres away and are small towns, so check the tank before you commit; Mantes-la-Jolie, about twelve kilometres off, has everything. Claude Monet's gardens at Giverny are not far upriver, so on a summer weekend this whole stretch carries coaches and cyclists, and patience on the narrow bends is part of the deal.