Tiantan isactually saint place in china history, where most of the emperors pay theirrespect to the heaven, make prayer, and requesting welfare for its 子民/庶民(不是公民citizen)
The round buildingis in 3 floors. Actually the round shape had a hidden meaning of how Chinese peoplethink of the world, the sky is round ,and the earth is square, which lasts for severalthousand years in ancient china.
The building isbuilt on high base, which brings respect of the people, and also importantlyavoid flooding.