Wongudan Altar, 1926

Found at the Marmot's Hole, a photo of Seoul City Hall and Wongudan Altar from 1926.  Wongudan, which means "Round Hill Platform", is Historic Item #157.   Taken from Chinese imperial practices, King Sejong the Great used the altar for sacrifices during a drought.  The use of the altar as a regular practice never caught on, though. 
Today Wongudan can be found behind the Westin Chosun Hotel.