I wandered a couple days ago into a big arena-like area in a public park in Beijing. It turns out it's the location where the Emperor (in Ming and/or Qing dynasties) would, once a year, offer prayers to the Sun.
There are four such parks -- one for Sun, one for Moon, one for Heaven, one for Earth. They're positioned in the four directions (with the Forbidden City = Emperor's abode, at center).
The Sun praying place was quite impressive. One imagines some serious geomancy may be involved.