I have a new question regarding of 72 Hour Visa-free transit

First One:
(Hey Gatcke! I'm afriad you are not eligible for the72 hour free transit because you want to go to two cities (Beijing and Xi'an).)

New question:
" Am I correct in that if I just visit the Great Wall that I will then be eligible for the 72 free transit? Just to confirm I will be traveling from Hong Kong on a South African Passport - is this OK? "