A city is a big crowded place that's full of small crowded places. That's pretty much how a city happens. So it seems strange that in many cities there's a place where people do not go.
Most cities call these bad neighborhoods and (because they are neighborhoods) people do go there. People, in fact, live there. The cities with bad neighborhoods are just over-reacting.
Other cities - the ones that are not over-reacting - have what we have to call Empty Zones. If you were to track the movements of all the people in those cities on a map, you'd see swirling vortices of I'm-not-going-there that neatly frame the Empty Zones in infinitely complex fractal patterns. These vortices have eddies and peninsulas and spiral valleys where people nearly went there, but thought better of it, or bounced off.
But if you were to look very, very closely at these maps, every now and then a small track would lead right into an Empty Zone. Because folks are just like that. The reason more folks aren't like that is this: most of these tiny traces don't come out again. So long as those adventurous few explore the Empty Zones before they've had a chance to make more people like themselves, well, natural selection does the rest.
Now Retropolis is a wonderful place to live, but it's not an exception to this rule.
If you were to map people's movements in Retropolis you'd immediately see the large and perilous vortex that centers on the Experimental Research District. All of the most interesting laboratories are located there. By statute. Because as dangerous as it may be to group them all together it makes it a lot easier to point at it and say "Oh, and by the way, don't ever go in there."
But on closer examination the Experimental Research District isn't really empty, because very fine threads enter the vortex and do, quite often, leave. The District houses a bustling population of scientists and lab assistants. There's a regrettable amount of turnover in that population, but its numbers remain pretty steady: if you live in there, you probably don't want to live anywhere else. Which is fortunate because, statistically, you won't.
So by examining it closely we can see that the Experimental Research District is not really an Empty Zone. To find a true Empty Zone in Retropolis you need to look much, much closer. There is one. It's just that it's very small.
It's so small that it wouldn't even show up on a conventional map, and it doesn't appear on one. It's basically a big room at the end of a tunnel at the bottom of a shaft, down a ladder, deep below the streets.
It doesn't need to be on a map. This is because of that 'natural selection' we discussed earlier. The residents of Retropolis no longer need to be told that they shouldn't visit the Clockwork Book. They're descended from the people who were smart enough not to.