Before there was art, there was defence. The Louvre began in the late twelfth century as a medieval fortress, built by King Philip the Second to protect Paris. Picture it as a working military structure rather than a showpiece: thick walls, a moat, a keep. That is the origin hidden beneath the elegance you see today, and it is one of the great surprises of this site. You can still find it. In the Sully Wing, the excavated walls of the old fortress are on display, and part of the medieval structure survives in a lower hall built in the thirteenth century. Standing there, with the stone a metre from your shoulder, is a genuine time-travel moment: the same ground that once held archers now holds visitors with headphones. The transformation came gradually. In the sixteenth century, King Francis the First rebuilt in a Renaissance style and, crucially, began collecting art with real appetite. Works he amassed in that century still form a core of the collection, including pieces by Michelangelo and Raphael, and the museum's most famous painting, Leonardo da Vinci's Mona Lisa. One charming detail of language: in French, a château traditionally suggests a grand house in the countryside, while a palace is the urban equivalent. That is why this is the Palais du Louvre, and Versailles, out in the countryside, is a château.