The chapel is 24 metres long by 10 metres wide. Its windows, uniquely for a Gothic building, are rectangular at the top rather than ogival. The corners of the ribbed vaults are decorated with seven heads sculpted in the round, and include Saint Louis and his mother, Blanche of Castile. A rose window, 10 metres square, once “adorned with painted glass”, occupies one of the walls. It was here in 1238 that Baudoin II, Emperor of Constantinople, gave the relics of Christ to Saint Louis, in particular the crown of thorns, later kept in the Sainte Chapelle in Paris built specifically for this purpose. Baptisms of royal children, both legitimate and illegitimate, and royal and princely marriages were held here in the following years. Louis XIV was baptised here, and this was where Francois I married Claude de France in 1514.