"Always said to have been built to repel Mongolian hoards and other invaders ... but on closer examination the arrow slits clearly point back into China in many parts of the wall!"
Actually, it's not one wall, but a whole number of different "walls", built at different times, over a period of some 2000 years overall. & some walls were not against steppe invaders, but against other Chinese states. What tourists nowadays usually visit are Ming Dynasty walls, restored & reconstructed with tourism in mind.
Sadly, the video doesn't contain a lot of valid information, it's mostly nonsense.