Jerusalem is builded as a city that is compact together:

Jerusalem, the builded one, Is as a city that is joined to itself together.

O Jerusalem, you are like a town which is well joined together;

Jerusalem is built as a city. Here David begins to celebrate the praises of Jerusalem; and he does this with the design of encouraging the people to persevere with uniform steadfastness in their obedience. It was of great importance for the minds of the godly, instead of being drawn hither and thither, to be kept constantly fixed on that city, which was the bond of a holy unity. When the people came to be divided into two bodies, that was the commencement of melancholy devastation. It is not surprising, then, to find David commending with such earnestness the place which God had chosen, knowing, as he did, that the prosperity of the Church depended upon the children of Abraham worshipping God there in purity, according to the appointed observances of the law; and next, upon their acknowledging the royal seat which the same God had erected there by his own authority, and had taken under his own protection When it is said that Jerusalem is built as a city, it is not to be understood as referring only to the walls, or towers, or ditches of that city, but chiefly to the good order and holy polity by which it was distinguished, although I allow that there is some allusion to its ancient state. Salem, indeed, had been a noted town even from the beginning; but when God selected it to be the head of the kingdom, it changed its appearance, and in a manner its nature, so that then it began to deserve the name of a well-regulated city. At first sight it may seem a poor commendation to call Jerusalem a city; but it is to be observed that it is here exhibited as it were standing alone in the whole world -- taking the precedence of all other cities, which will in vain attempt to equal it. David, certainly, in thus speaking, does not intend to divest other cities of the rank to which they may be entitled, but he raises Jerusalem higher, that it may appear conspicuous above them all, even as we find Isaiah, (Isaiah 2:2,) when speaking of mount Zion, asserting that it "shall be established in the top of the mountains, and shall be exalted above the hills." In that passage the Prophet, to magnify this little hill, brings down the loftiest mountains of the world, that they may not obscure its glory. In like manner David here affirms that Jerusalem is compacted as a city, to induce the faithful, instead of gazing in all directions around them, to rest contented with the city which God had chosen, since they would nowhere find its equal. After having humbled all other cities, he shows, in a few words, the excellence of Jerusalem, representing it as regularly built, or fitly and neatly joined together in all its parts. Some take these words as expressing literally and without figure, that its citizens live together in peace and unity; but I see no impropriety in supposing that they describe, metaphorically, the peaceable state of a city. Thus the mutual concord which reigns among the citizens of a city, and by which they are united to each other, is compared to buildings, compacted together by a skillful and elegant workmanship, so that there is nothing imperfect, in joined together, or rent, but throughout a beautiful harmony'. By this David teaches us, that the Church can only remain in a state of safety when unanimity prevails in her, and when, being joined together by faith and charity, she cultivates a holy unity.

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.

Jerusalem - compact together - It is now well rebuilt, every part contributing to the strength of the whole. It is also a state of great political and spiritual union. It is the center of union to all the tribes, for each tribe has an equal interest in that God who is worshipped there.

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.

Jerusalem is builded as a city that is compact together. In David's time the upper and lower city were joined together, the streets regularly built, the houses contiguous, not straggling about, here and there one (c). So the church of God, like that, is built in a good situation, on a rock and hill, where it is firm and visible; like a city full of inhabitants, governed by wholesome laws, under proper officers; a free city, which enjoys many privileges and immunities; a well fortified one, having salvation for walls and bulwarks about it; a royal city, the city of the great King, the city of our God, the name of which is "Jehovahshammah", the Lord is there: and this is "compact together" when its citizens are united in affection to one another; agree in their religious sentiments; join in social worships, and live in subjection to one Head and King, the Lord Jesus Christ. The Jews often speak, and so some of their commentators on this passage, of a Jerusalem above and below, and of the one being made like unto the other: so the Targum,
"Jerusalem is built in the firmament as a city, as Jerusalem on earth;''
see Galatians 4:26.
(c) Hecataeus, an Heathen writer, describes Jerusalem as a strong fortified city, fifty furlongs in circumference; and inhabited by twelve myriads, or a hundred and twenty thousand men. Vid. Euseb. Praepar. Evangel. l. 9. c. 4.

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