Since you mention a trip to Beijing, I wonder what the security precautions were to keep the secret base secret. I assume visitors from other cities would need to apply for a travel permit similar to the one still required for some border areas in Xinjiang and Tibet, but were there also restrictions on people leaving?

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.