Jerusalem is builded as a city that is {c} compact together:

(c) By the artificial joining and beauty of the houses, he means the peace and love that was between the citizens.

Jamieson-Fausset-Brown Bible Commentary

3-5. compact together-all parts united, as in David's time.

Matthew Henry's Concise Commentary

122:1-5 The pleasure and profit from means of grace, should make us disregard trouble and fatigue in going to them; and we should quicken one another to what is good. We should desire our Christian friends, when they have any good work in hand, to call for us, and take us with them. With what readiness should we think of the heavenly Jerusalem! How cheerfully should we bear the cross and welcome death, in hopes of a crown of glory! Jerusalem is called the beautiful city. It was a type of the gospel church, which is compact together in holy love and Christian communion, so that it is all as one city. If all the disciples of Christ were of one mind, and kept the unity of the Spirit in the bond of peace, their enemies would be deprived of their chief advantages against them. But Satan's maxim always has been, to divide that he may conquer; and few Christians are sufficiently aware of his designs.