The smaller, older of the two buildings that make up the Château de Chantilly. It was built in the 1560s by Anne de Montmorency, Constable of France, hence its alternative name the Capitainerie. Unlike the Grand Château, it survived the Revolution largely intact — its smaller scale and humbler architecture made it less of a political target.
Inside, the Petit Château holds the private apartments of the Princes of Condé: a chapel, a music room, the Singerie (a small room decorated with monkey-themed paintings, a kind of 18th-century joke at the expense of aristocratic portraiture), the Library (about 19,000 volumes including the Très Riches Heures manuscript safe), and the impressive Galerie des Batailles, lined with paintings commemorating the military victories of the Grand Condé.
The Petit Château is included in the standard château ticket. Don't skip it on the way through to the museum proper — the rooms are smaller than the picture galleries but more atmospheric, and the Singerie alone is worth a quiet ten minutes.