Circular Mound Altar
South of the Echo Wall stands the Circular Mound Altar, which is wholly built of white marble. The two separate walls-enclose the altar. A square wall is outside and a round one being inside.  Its layout forms a geometrical pattern but the structure itself is so designed as to make the most ingenious use of geometry. The altar is 5 metres high.  The upper terrace is 30 metres in diameter, the middle terrace 50 metres and the bottom terrace 70 metres. Each terrace has four entrances and a flight of nine steps leading down in every direction. At the centre of the upper terrace lies a round stone surrounded by nine concentric rings of stones. The number of stones in the first ring is 9, in the second 18, and so on, up to 81 in the 9th ring.
   The middle and bottom terraces also have 9 rings each. The total number of the marble slabs paved on the altar's surface is 3,402.  Even the numbers of the carved balustrades on these terraces are also the multiples of 9.  The number of the balustrades is 360, which stands for the 360 degrees of the circumference of heaven.  When you stand in the centre of the upper terrace and speak in a low voice, your echo will sound much louder to yourself than to others, as the sound waves are sent back by the marble balustrades to focus at the centre. The altar was built in 1530, and rebuilt in 1740 in the Qing Dynasty (1644-1911). Although it is over 200 years old, the structure still stands perfectly erect without sagging or cracking.
  Every year on Winter Solstice, escorted by soldiers and officials, by musicians and by princes of the royal blood, the emperor would come here to make solumn sacrifice. From the Front Gate to the entrance of the Temple, each gate and every window overlooking his route had to be closed tight. No foreigners were allowed to watch the procession and foreign diplomats were officially advised to stay in their quarters that day. It even happened that the train from Tianjin was forced to stop so as not to disturb the silence of the ceremony
    The emperor spent a night of fasting and praying in the Temple. The King of Zhou, claiming to be the son of Heaven, made the first offer to Heaven 3,000 years ago.  Yuan Shikai (1859-1916), a warlord, made the last sacrifice to Heaven in 1914. Southeast of the Circular Mound Altar stands an oven built of glazed tiles.  It was used as a sacrificial vessel for roasting ox and sheep, etc.
  Southwest of the Altar is a viewing lantern platform, on which a big pole was fixed for hanging big lanterns during ceremonies.