The Château de Saint-Germain-En-Laye is a royal palace in the commune of Saint-Germain-en-Laye, in the departement of Yvelines, about 19 km west of Paris, France. Today, it houses the Musée d'Archéologie Nationale (Museum of National Archaeology). The first castle, named the Grand Chatelet, was built on the site by Louis VI in around 1122. The castle was expanded by Louis IX of France in the 1230s.
Louis IX's chapelle Saint Louis at the castle belongs to the Rayonnant phase of French Gothic architecture. A 1238 charter of Louis IX instituting a regular religious service at the chapel that we first learn of a chapel having been built at the royal castle. This was a Sainte Chapelle, to house a relic of the Crown of Thorns or the True Cross. Its plan and architecture prefigure the major Sainte-Chapelle which Saint Louis built within the Palais de la Cite at Paris between 1240 and 1248.
Both buildings were built by Louis's favourite architect Pierre de Montreuil, who adapted the architectural formulae invented at Saint Germain for use in Paris. A single nave ends in a chevet, with almost all the wall areas filled by tall thin glass windows, between which are large exterior buttresses. The castle was burned by the Black Prince in 1346; of it, and only the Gothic chapel remains from the site's medieval phase. This Château Vieux was rebuilt by King Charles V in the 1360s on the old foundations.