Jerusalem is built as a city that is compact together:
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Augustine of Hippo
4. "Jerusalem that is being built as a city" (ver. 3). Brethren, when David was uttering these words, that city had been finished, it was not being built. It is some city he speaketh of, therefore, which is now being built, unto which living stones run in faith, of whom Peter saith, "Ye also, as lively stones, are built up a spiritual house;" that is, the holy temple of God. What meaneth, ye are built up as lively stones? Thou livest, if thou believest: but if thou believest, thou art made a temple of God; for the Apostle Paul saith, "The temple of God is holy, which temple are ye." This city is therefore now in building; stones are cut down from the hills by the hands of those who preach truth, they are squared that they may enter into an everlasting structure. There are still many stones in the hands of the Builder: let them not fall from His hands, that they may be built perfect into the structure of the temple. This, then, is the "Jerusalem that is being built as a city:" Christ is...
Together. Well built and inhabited. The Jews throughout the world considered it as their most dear country. (Calmet)
The participation of spiritual graces (Psalm cxviii. 63.) is a great comfort to Catholics, (Worthington) who look upon the chair of St. Peter at Rome as the centre of unity. (Haydock)
David established order in Jerusalem, when he had made the conquest, and various families contributed to the rebuilding of it. (Berthier)
All the tribes were united under the dominion of David. (Houbigant)