Charles Ellicott Commentary Psalms 122:3

"Jerusalem, that art builded As a city that is compact together;" — Psalms 122:3 (ASV)

This verse is somewhat perplexing. One explanation is that it refers to the rebuilding of the city and the reuniting of parts that had been disconnected during its destruction. Another interpretation, which is far better (see Introduction), is that it is a rustic’s impression on first seeing a compact city after being accustomed to straggling villages. The astonishment of Virgil’s shepherd is aptly compared: “Urbem quam dicunt Romam, Melibæe putavi, Stultus ego, huic nostræ similem.”

But a far more satisfactory meaning is suggested by the Septuagint. They (compare Symmachus) take the word rendered compact as a noun, meaning union. The verse then may run: Jerusalem, the (one) built like a city, union is in it together, i.e., it is the rallying point of all the tribes. (See next verse.)