Eruvin 6: Lock Up Your City at Night

Terminology: (Mavoi) Mefulash. Also Mavoi Akum. We start with the premise that the in-between status of alleyways as a karmelit will be next to a public domain, and expand the private domain into the karmelit. But this daf suggests that an eruv could be made in a public domain. Could that be done with a lechi (sidepost)? Perhaps you need actual doors at every exit point? Jerusalem becomes a counterpoint - where the doors (gates) were locked at night. Similarly, Mehoza (another city). Those locks seem to make all the difference. But that what happens with alleyways that open into the public domain? That needs a "tzurat ha-petach," lechi, and koreh. Challenge case of Manhattan - a very complicated place to make an eruv, if you can at all. Note that Eruvin is a lot of real-life cases, perhaps less exploratory for the sake of the parameters of halakhah.