The Temple was built in 1420 A.D. during the Ming Dynasty to offer sacrifice to Heaven. As Chinese emperors called themselves "The Son of Heaven" ,they dared not to build their own dwelling,"Forbidden City" bigger than a dwelling for Heaven.
Religious - Buddhism
The Temple of Heaven is enclosed with a long wall. The northern part within the wall is semicircular symbolizing the heavens and the southern part is square symbolizing the earth. The northern part is higher than the southern part. This design shows that the heaven is high and the earth is low and the design reflected an ancient Chinese thought of "The heaven is round and the earth is square". (travelchinaguide.com)