Donjon du Château de Vincennes, Medieval keep at Château de Vincennes, France

The Donjon of Château de Vincennes is a square keep of pale beige stone rising 50 metres above the courtyard, with round corner buttresses capped by projecting cornices and a flat top flying the French flag. Narrow slit windows pierce the walls, and a lower gatehouse with two round towers and a steep hipped roof stands in front. Inside, a chamber of dark stone blocks is covered in dense cross-hatching, with iron rings and hanging chains; a man sits chained on a stone bench beside a wooden chest and a bucket.

Construction began in 1361 under King Charles V and transformed an original hunting lodge into one of the tallest fortified structures of the Middle Ages. The building remained a center of royal power for centuries and was later used as a prison.

The name donjon comes from the Latin dominium, the lord's tower. In the 18th century the keep held a small staff for prisoners' welfare: a doctor, a surgeon, an apothecary, a dentist, an oculist, a confessor and a kitchen team, while three guards served inside.

The keep stands in the castle courtyard, reached from the château entrance; its rooms are stacked on several floors around a spiral stair, so expect steps and uneven stone floors. The 8-sided chambers of the corner turrets, used as cells in the 18th century, are the smallest spaces to visit. A door and stove brought from the Temple in 1808 by Napoleon are still shown inside.

Diderot was not held in the keep itself but in a now-demolished building next to the Sainte-Chapelle. The keep's best-known escape was that of François de Vendôme, duc de Beaufort, who at Pentecost 1648 climbed down a rope from the curtain-wall gallery into the moat, where 50 mounted helpers waited; the plot failed. Napoleon had a prison door and a stove from the Temple installed in 1808, still visible inside.