That’s a great question. In the early days, physical travel permits were indeed the norm. But the most effective 'security precaution' was psychological.
We had secrecy education (保密教育) starting as early as primary school. We were taught from a very young age that our city didn't exist to the outside world, and we simply didn't talk about it. But when I was a kid ,I didn't know anything about 404.
We had secrecy education (保密教育) starting as early as primary school. We were taught from a very young age that our city didn't exist to the outside world, and we simply didn't talk about it. But when I was a kid ,I didn't know anything about 404.