Who built this mysterious, ancient castle, nestled deep within the mountains?

Deep within the desolate mountains of Hebei Province, a unique architectural structure has been discovered. Built into the side of a mountain, it resembles both a mysterious ancient military fortress and the Potala Palace of Tibet. Nestled among the mountains, this ruined structure clearly shows its age and long history. From above, numerous houses of various sizes can be seen, some even equipped with solar panels to generate electricity. Surrounded only by barren loess hills, the area is devoid of life, suggesting extremely harsh living conditions. Who once lived here? And who built it? With these questions in mind, we set out to investigate.