After 1421 Beijing became China's capital, and its group of imperial buildings, known as the Forbidden City, remains a remarkable achievement. Around its main courtyard and many smaller courts are grouped splendid halls, galleries, terraces, and gateways. White marble, wall facings of glazed terra-cotta, roofs of glazed and colored tiles, and woodwork finished with paint, lacquer, and gilding unite to create an effect of exceptional richness. Notable among these buildings is the group constituting the Temple of Heaven, including the Hall of the Annual Prayers (added in the late 19th cent.), a circular structure on a triple platform surmounted by a roof in three tiers covered with tiles of an intense blue glaze.