Keep of Vincennes

This guide to the Keep of Vincennes in Paris opens on a July morning in 1749, when the writer Denis Diderot is arrested for a slim book and marched into Europe's tallest medieval fortress tower. Locked in a cold stone cell, he's kept from total isolation by a friend who walks six miles from Paris almost daily to see him. That friend is Jean-Jacques Rousseau, and the guide follows him along the dusty road that changes the course of his life. Picture the walls, over three metres thick, that once caged some of the era's most dangerous minds.