Why is the yamen open in eight characters?

In ancient times, almost all official doors were two brick walls lined up in a line, extending obliquely to the left and right, like a figure of eight. The door is open and not closed, and the brick wall seems to be an extension of two door panels, which is the origin of the "eight-character open yamen". Until now, this architectural form can still be seen in some streets or old houses in hutongs in Beijing.