Château De Saint-Jean-D'Angle.

You get dressed before you cross the bridge. Costumes are free at the gate — knight, page, castellan — and the effect is unreasonable: a curtain wall traditionally dated to around 1180 stops being a photo subject and becomes somewhere you're standing inside. Tradition credits Guillaume de Lusignan and Denise d'Angle, though no charter survives to prove it. What the Ministry's own record does confirm is the shape: a polygonal ring wall on a motte, no central keep, lodgings ranged in an L around the courtyard, moat all the way round.

Climb to the chemin de ronde and the reason for all of it lands. No mountain to watch, no river crossing, no pass. Only an ocean of marsh — some 6,000 hectares of salt pans stretching toward Brouage, ten kilometres off. Salt was the sole way to carry fish and meat through a European winter, and this wall stood over the road it left by. The castle isn't a castle in a view. It's a tollbooth on a commodity.

And it very nearly went. When the state classified it on 21 March 1994 the place was roofless and half-swallowed by brambles, stone sliding out of the ramparts. Alain Rousselot bought the ruin that December and spent 1995 to 2000 putting it back — 76,000 litres of lime-and-water grout, enough to fill a small swimming pool, poured into every cavity in the wall. Europa Nostra gave the work a heritage award for it. His line on the result: over 800 years old, and properly maintained it will still be here in another 800.

Most visitors walk straight past the finest carving here. The doorway into the 15th-century logis is flamboyant Gothic done properly: colonnettes, pinnacles, an accolade curling to a point, and above it a heraldic shield held up by two lions. Shoot it from below or you flatten the whole thing into wallpaper.

Then go looking for the amateur work. A survey of the walls records anchors and boats, crosses, a church steeple — cut into the logis, the tower and the latrines, exactly the parts still reachable during the centuries of abandonment. Boats, this far inland, because navigation was what everyone here did for a living. And fleurs-de-lis inside the privy and around its door. Royal emblem, in a toilet, carved by people whose names nobody bothered to write down. That's a single published survey, so ask at the desk before you go hunting.

The lines are shallow and softened by weather. They read in raking light or not at all, which makes early morning and late afternoon the only honest times to look; run your fingertips across the jamb instead of staring at it head-on. While you're below courtyard level, take the stair out of the larder — a shoulder-width shaft you turn sideways in, twenty metres of wall overhead, and then daylight. Best thirty seconds in the building.

Do it in this order and the day makes sense. Costume at the gate, then the stone castle while the sun is still low and the graffiti still legible — courtyard, guard room, the deep pit, the latrines, the larder stair, the rampart walk last for the marsh panorama. Reckon on an hour before you've had enough of walls.

Then walk out to the timber motte, the reconstructed fort of the year 1000 on its earthen mound, and turn around. Wooden palisade in front of you, stone shell wall behind: two centuries of military technology in one sightline, and the clearest lesson on the whole 15-hectare estate. From there the medicinal garden's twelve planted squares, the Aliénor rose garden, and the barefoot path — shoes off, and yes, do it. The musical forest is the one genuinely odd thing on site.

Time the trebuchet demonstration and don't miss it; the release is loud, the landing louder, and the machines are the last working ones in Charente-Maritime. Note they're cancelled outright during extreme-heat alerts, and a mound with no shade in August is exactly as hot as that sounds. Budget three hours. Adults €15, children 4 to 13 €12.50, under-fours free, and the ticket office shuts an hour before the site does — arriving at four in the afternoon buys you a rushed castle and nothing else.