The Dark Castle of the title refers to the fortress at Khara-khoto, which was a main center of the Tangut state. The Castle has legendary status amongst the local people, who fear to go there, but the crew find some guides and trek across the desert on camels.
This was a Buddhist Kingdom, following Tibetan-like traditions, the remains of the fortress are strewn around, and they come across scraps of scriptures, silk, tiles, buttons, and other remains.
This episode also recounts the battle between the Tangut King Batir and the armies of Genghis Khan, which ended which the destruction of the King and the Kingdom.
(the comment is not published)