In the course of the discussion I learned a new (to me) term - 塞墙。sai4 qiang2
That character sai4 translates as “a place of strategic importance”; qiang2 just means wall. I verified with him he meant sai4 and not sai1 which wouldn’t make sense anyway.
The question came about when I asked had he looked at the walls connecting Yangguan to Yumenguan. He said there were no such walls. Through the conversation it became clear that he differentiates between 长城, referring to what he called the big walls designed to stop anything getting past, and 塞墙 which he says were designed to impede the movement of wagons carrying looted supplies. He said 车 and I pointed to a wagon and he nodded, so that is what he meant. Each, or most, 塞墙 had a ditch related to it, to enhance the obstacle.
Another person there said that after the Qin dynasty, subsequent dynasties associated Qin’s （万里)长城 with the demise of the dynasty, so to avoid a similar fate they referred to their defensive walls as 边墙。
This accords with the words of Wan Sitong, a Qing Dynasty historian:
The men of Qin built the Long Wall...and down came the Empire
People are still laughing about it today.
Why did we build walls for 10,000 li?
Dynasty after dynasty has done the same thing
So why do we laugh at the First Emperor of Qin?
According to Mr Zhang, the term 塞墙 applied to the low remnant wall that runs alongside (and crosses at 39