There were big battles in those days at Gubeikou and at Nankou (basically along the whole stretch between about Badaling and Shuitoucun).
What is particularly interesting is how many towers were fairly intact back then, but are ruined now. I've often thought, especially at Gubeikou, that one possible explanation for the good condition of one tower next to the complete ruins of another could be that towers were hit by artillery during those battles. Maybe I'm right, given that so many of them were still apparently relatively intact when these images were taken (meaning they'd survived a few centuries of potential looting by villagers).
Well worth a look and you don't need to be able to read Chinese to enjoy it.