Although Zhao Xi had only perfected the first nine layers of the Eighteen Layers of Heaven's Cultivation Method, even he had only cultivated up to the Sixth Layer Heaven.
But...
The statement that it took him sixteen hundred years to cultivate to the Sixth Layer Heaven is not entirely accurate.
During the first sixteen hundred years, Zhao Xi's progress was gradual, and it's likely that his Spiritual Realm had already reached the equivalent of the Fourth or even Fifth Layer Heaven.
It was only in the last few decades that he came across a Cultivation Method related to Divine Sense on Blue Star, which allowed him to perfect the system of Spiritual Realm and create the first nine layers of the Eighteen Layers of Heaven's method. He personally practiced it and completed the cultivation of the Sixth Layer Heaven.
