Albert Barnes Commentary Psalms 122:3

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

Jerusalem is builded as a city that is compact together - literally, “joined to itself together;” that is, when one part is, as it were, bound closely to another part, not scattered or separate. The walls are all joined together, and the houses are all united to one another so as to make a compact place.

The ground occupied by Jerusalem never could be large, as it was surrounded with valleys, except on the north, and hemmed in with hills, so that, from the necessity of the case, when it became the capital of the nation, it was densely crowded. This, moreover, was usual in ancient cities, when they were made compact for the sake of defense and protection.