How was Gyeongbok Palace's drainage system built?

Even when torrential rain pours down, water doesn't pool on the stone courtyard in front of Gyeongbok Palace's Geunjeongjeon Hall. This is despite water flowing directly from Baegaksan Mountain behind the palace. If they blocked it with walls, the water inside would be trapped, turning the entire palace into a basin, and if they tried to channel it out in one stream, the water's force would erode the floor stones. So, Joseon chose to break it down into smaller streams and let it flow slowly. They created separate drainage channels for each area, with sumps called 'jipsujeong' to settle dirt and fallen leaves before the clear water was directed into underground stone waterways. What's most surprising is that they didn't excavate the land but utilized its original slope to guide the water's path. [][]