Originally Posted by
Ian Liew
Wanli was a Ming dynasty emperor - he reigned for around 40 years, the 1st half being a good emperor with 20 years of development, and his later 20 years being terrible. He was so corpulent that he couldn't walk and had to be carried. Wanli irrevocably set in chain the events which would destroy the Ming dynasty by the time Chongzhen inherited the throne, as both Taichang and Tianqi did nothing to stop, and probably even hastened, the slide.
The Emperor must have known that his guards were all going to be away that night (you couldn't just leave the palace empty without informing his majesty). Wei Zhiyun and his friends might have given him a detailed report on who was dueling, their background, their skills, the significance of the duel and why all the guards HAD to go and watch this once-in-a-lifetime match. It says a lot for the Emperor's self-restraint (or his lack of interest in martial arts) that he wasn't on the rooftop himself, really.