The number ‘9’ being the largest single digit number, represents emperor/royalty in the Chinese culture. And of course ‘8’ in China is considered a very lucky number as it sounds like the word ‘prosperous’ . The reason for 17 arches is that from the central arch (9th arch) there are 8 arches on either side, representing eternal prosperity of the Chinese emperor.
It is really the ‘Great Wall’. Built over 100 years by successive rulers; nothing permeates the psyche of the Chinese as the Great Wall. It has given them a sense of protection and security, even though it may not have been used to repel any attacks directly. May be, this wall has also made them more closed to the outer world than others.
Whatever said, it is a marvel, a sheer triumph of human endeavour and engineering over adversities of nature.
ps: I trekked the entire length visible in this picture. From one peak to another. Totally may be 1500 steps. It was worth it!