Construction on the Temple of Heaven began in 1406 during the reign of Emperor Yongle (r. 1402-1424) and took 14 years to complete. The complex contains three main buildings where the emperor, as the 'Son of Heaven', went during the winter solstice to offer prayers and sacrifices for a good harvest. The emperor spent the night before the ceremony fasting in the Hall of Abstinence. The Hall of Prayer for Good Harvests in the north stands 39 m tall and is supported by 28 wooden pillars covered with three conical roofs, all covered with blue glazed tiles. The last person to use the hall was Yuan Shi-kai, president of the new republic, who had imperial ambitions and who offered imperial sacrifices on the winter solstice in 1914. The Hall of the Imperial Vault of Heaven, in the centre, stored the ceremonial tablets used in rituals. Echo Wall, a circular brick wall surrounding the Imperial Vault, has the acoustical ability to enable two people standing at opposite points to hear each other whisper. The Circular Mound Altar of Heaven, to the south, is where the emperor offered sacrifices and prayed.