Our Rabbis taught: How is a road through a public domain to be provided with an eruv? The shape of a doorway is made at one end, and a side-post and cross-beam, [are fixed] at the other.
This is a major road – 16 cubits wide – going all the way through a major town of 600,000 or more, and no wall around. The road itself needs entrances in order to be considered “contained.” The school of Shammai, as usual, made a more restrictive ruling:
A door is made at the one end as well as at the other and it must be locked as soon as one goes out or enters
And the primary example:
did not Rabbah b. Bar Hana state in the name of R. Johanan that Jerusalem, were it not that its gates were closed at night, would have been subject to the restrictions of a public domain
It makes me think of the communal effort that would have to dedicated to make the entire city a “private” domain by way of this eruv. I have walked through those massive gateways in the Old City of Jerusalem, and think about the commitment to build for Shabbat.