What Makes an Eruv Valid Under Halacha
An eruv does not begin with a wire. It begins with the halachic status of the streets, continues with a complete enclosure suited to those streets, and includes the communal arrangements and supervision that allow the public to rely on it from one Shabbos to the next.
The purpose and mitzvah of an eruv
The Alter Rebbe writes that it is a mitzvah to pursue an eruv. In the cited halachos, the subject is eruv chatzeiros and shitufei mevo'os: the joining of residents within an area already enclosed according to halacha. The shared food removes the rabbinic restriction between their homes, courtyards, and shared lanes.
A city perimeter answers an earlier question. Before residents can be joined, the streets themselves must be enclosed in a form that works for their halachic status. The mitzvah of eruv does not turn an unsuitable boundary into a valid one; it is fulfilled by arranging the right eruv in the right place.
The status of the streets comes first
A karmelis can ordinarily be enclosed with tzuras hapesach. A full reshus harabim presents a different problem: under the halacha followed by the Alter Rebbe, ordinary doorframes are not enough, and the public ways require walls and gates in the prescribed form.
That is why width, openness, public access, the street network, and shishim ribo are considered before the route is designed. If the streets are misclassified, even beautifully constructed poles and wires do not answer the central question.
One continuous physical enclosure
A city boundary may combine actual walls, fences, riverbanks, embankments, building lines, and tzuras hapesach. Each section must possess the halachic form being claimed for it, and the sections must meet without leaving the enclosed area open at a corner, roadway, bridge, tunnel, or change of grade.
Existing structures are not counted by appearance alone. Their height, slope, continuity, openings, visibility, and relationship to public passage determine whether they serve as walls. A sound section on one side cannot close an untreated breach on another.
The communal joining
After the enclosure is established, eruv chatzeiros or shitufei mevo'os joins the residents through food acquired on their behalf. Sechiras reshus may be required for domains not included through the ordinary joining.
These are real parts of the eruv, but they do not create walls. Shared food cannot close a street opening, and a municipal rental cannot change a Torah-level public domain into a private domain.