Why is the Forbidden City also called the Forbidden City?

The Forbidden City is the palace of twenty-four emperors in Ming and Qing Dynasties. Astronomers in ancient China once divided the stars in the sky into three constellations: three walls, four elephants and twenty-eight constellations. The three walls include Taiwei Garden, Ziwei Garden and Tianshi Garden. The purple element is in the middle of the ternary star, so it becomes the constellation representing the celestial emperor. The place where the Emperor of Heaven lives is called Purple Palace. The feudal emperor was honored as the "son of heaven". The palace is crowned with the word "purple" to show that it is the "purple palace" on earth and the supreme place on earth. In addition, since the Qin and Han Dynasties, people used to call the palace "Forbidden Palace", which means "the door is forbidden and you are not allowed to enter casually". The word "purple" and the word "forbidden" are combined, so the palace is called the Forbidden City.