The Hall of Prayer is a lofty, round structure with triple eaves  and a cone-shaped deep blue-tile roof crowned with a gilded knob. Surrounding the hall is a spacious circular stone terrace at three levels,  each laced  by a balustrade of  carved white marble.  The construction of the hall  is ingenious,  the inner frame  devoid of steel, cement, or beams  supporting the entire structure--38 meters high & 30 meters in diameter  on  28  massive  wooden  pillars.  The four central columns,  called  the "dragon-well pillars", are 19.2 meters high with a circumference of two and a half persons at  arms  length.  These columns  represent  the  four  seasons.  They are surrounded by two rings, one inside the other, of 12 columns each. The inner ring symbolizes the 12 months of the year,  the outer the 12 divisions of day and night according to the old Chinese way of reckoning time, that was in 2 hours periods, making 12 units.  The timber for these 28 columns  was transported from distant Yunnan Province  in  southwest  China.  In the center  of  the flagstone floor  is a round marble slab marked by nature with a dragon and phoenix pattern. 

The raised walk leads south to the Imperial Vault of Heaven. The Vault of Heaven is surrounded by a circular wall of polished brick with an opening to the south. This is known as Echo Wall, because if a person whispers close to the wall at any point, the voice can be heard distinctly at any point along the wall. 
South of Echo Wall  lies  Circular Mound Altar, a three-tiered white stone terrace enclosed by two walls, the inner one round & the outer one square. The structure itself  is a geometric game.  In the old days,  the odd numbers  were  regarded as "heavenly".  Since  9  was considered   the most powerful number,  the altar  was constructed with 9 or multiples of 9 stone slabs. The terrace is made up of 9 rings of stone slabs.  The innermost ring on the top consisting of 9 slabs, the second of 18,  the third of 27  and so on  to the 9th ring,  which  consists of  81 slabs.  This scheme is repeated on the middle & lower tiers, the rings increasing in size till the largest ring  at the bottom  is made up of  243  stone slabs.  Standing  on  the top terrace,one has the feeling of being very close to the sky. If you stand in the center of the top terrace and  speak with a normal volume, your voice will sound louder and more resonant to yourself than to others standing around, because the sound waves reflected by the balustrades & the round wall bounce back to the center.