Free Paris, Paris: Journey Into the City of Light by David Downie

books is that the 1659 townhouse capping the Quai de Bourbon goes by the name “House of the Centaur,” because of the pair of low-relief sculptures on the façade showing Hercules fighting Nessus, the savage mythical beast, half man, half horse. For years Madame Louise Faure-Favier held her literary salon here, hosting poet Guillaume Apollinaire, painter Marie Laurencin, writer Francis Carco, and poet Max Jacob, Picasso’s penniless friend. The centaurs overlook a pocket-size park, which is a popular picnic and panoramic spot. Alison and I come here often for the view. The mansion’s current occupants apparently enjoy entertaining. On more than one occasion we have watched as society women in gowns and gentlemen in tuxes mingled under a second-floor ballroom’s painted ceiling.
    It was this kind of wordly tableau that inspired eighteenth-century author Nicolas-Edme Restif de la Bretonne to invent a new literary genre, the nighttime prowl, writing about it from 1786 onward in Les Nuits de Paris ou Le Spectateur nocturne , a rambling account of 1,001 nights on Paris’s streets. His ramblings often started from the Île Saint-Louis (he lived nearby). Taking nighttime walks around the island, with a magic-lantern show of interiors, is my preferred form of voyeurism.
    Another, equally fun by day or night, is to poke around the shady courtyards of the island’s largely impenetrable mansions. Electronic coded locks called Digi-codes keep the rabble out. But I’ve discovered two methods to subvert them: wait outside and when someone leaves, confidently stride in, or, two, follow local mail carriers with passkeys on their rounds, starting about ten a.m. Stealth and subterfuge transform innocent exploration into an adventure. They once got me into number 15 Quai de Bourbon after years of cat-and-mouse with the concierge. Hidden in the wide cobbled courtyard I discovered a stone staircase with elaborate ironwork railings. On the roof above rises a two-story gable fitted out with a pulley—presumably to hoist furniture or intruders like me.
    By systematically testing the island’s doors I’ve found a few that are nearly always open. The best belongs to the Hôtel de Chenizot, at number 51 Rue Saint-Louis-en-l’Île. Giant griffins uphold a balcony over the studded door. Step in and at the back of the first crumbling court you see a low-relief Rococo floral burst. Keystones carved with heads peer down at you. The rusticated sections of the mansion date to the 1640s. The taller additions are from 1719, when Jean-François Guyot de Chenizot, a royal tax collector from Rouen, redecorated. The building then spiraled downward, becoming, in succession, a wine warehouse, the residence of Paris’s archbishop, a gendarmes’ barracks, a warehouse again, and a moldering apartment house. Today the leprous plaster hides the requisite sweeping staircases and a rear court with a weathered sundial. Though parts of it have been scrubbed, atmosphere still oozes from the place like wet mortar between bricks.
    There is a third method I have mastered for penetrating the isle’s inner sancta: take a guided tour. This can turn mystery into mere history, but it’s the only way to get inside the sole townhouse whose interior is accessible to the public, the Hôtel de Lauzun. Here you actually taste a crumb of the upper crust’s lifestyle under the Bourbon Louis—numbers XIII to XVI. The building’s history is lavished upon you, like it or not, by a loquacious cicerone. A potted version of it might run as follows.
    Charles Chamois, a military architect, designed the Hôtel de Lauzun in 1657 for a stolid cavalry commissioner named Grüyn. But no horses are to be found: Grüyn’s boar’s-head coat of arms shows up on fireplaces and wall decorations. Much racier a character, the Duc de Lauzun lived here from 1682 to 1684, shacked up with Louis XIV’s first cousin, alias La Grande Mademoiselle. That’s why the Lauzun name stuck.
    Surprisingly