Blessed be Jehovah! These general truths the Psalmist here proceeds to apply to his own circumstances, and he declares that the goodness of God in preserving his life was wondrously displayed. As he speaks of aid which had been suddenly and unexpectedly afforded him in very desperate circumstances, those interpreters judge aright who here supply as, the mark of similitude, [1] in this way, as in a fortified city David lay open to every blow, and had been exposed to every sort of injury, and he boasts that in his nakedness and destitution the assistance of God had been of greater service to him than a city well fortified, or an impregnable fortress would have been.

Footnotes

1 - "The particle of similitude is wanting in Hebrew, as is not uncommon. The intention of the Psalmist is evidently to describe by a metaphor his signal deliverance, as if he had been guarded by invincible fortifications." -- Walford.

Blessed be the Lord - An expression of thanksgiving for the evidence that God had heard him in his troubles, and had answered him.
For he hath showed me his marvelous kindness - literally, "He has made his mercy wonderful;" that is, he has showed me such mercy as to be an object of admiration and astonishment. It was not ordinary kindness, such as is shown to people every day; it was so uncommon - so far beyond all expectation - so separate from second causes and the agency of man - so marked in its character - as to fill the mind with wonder.
In a strong city - Margin, "fenced city." This may mean either that he had thus placed him literally in a strongly fortified city where he was safe from the fear of his enemies; or, that he had interposed in his behalf, and had given him protection as if he had brought him into such a strongly fortified place. Jarchi supposes that the city of "Keilah" 1-Samuel 23:7 is here intended. But this is improbable. All that the passage necessarily implies is, that God had given him protection as if he had been placed in a strongly fortified town where he would be safe from danger.

In a strong city - If this Psalm was written by David, this must refer to his taking refuge with achish, king of Gath, who gave him Ziklag, a fortified city, to secure himself and followers in. See 1-Samuel 27:6. This is more likely than that it was Keilah, where he only had intimation of the traitorous design of the inhabitants to deliver him up to Saul; so that the place was no refuge to him, howsoever fortified. Perhaps the passage may mean that, under the protection of God, he was as safe as if he had been in a fortified city.

Blessed [be] the LORD: for he hath shewed me his marvellous kindness in a (p) strong city.
(p) Meaning, there was no city strong enough to preserve him, as the defence of God's favour.

Blessed be the Lord,.... A form of thanksgiving, in which the psalmist calls upon himself and others to bless and praise the Lord for the singular mercy granted him, expressed in the next clause; See Gill on Psalm 18:46;
for he hath showed me his marvellous kindness in a strong city; either in the city Keilah, so Jarchi; a city which had gates and bars, where Saul thought he had David safe, and he could not escape his hands; but notwithstanding that, and though the inhabitants of that place intended to deliver him up, yet he was marvellously saved; as he also was from the Ziphites; and when Saul and his army had encompassed him about, by a surprising incident, a messenger coming to Saul just as he was about to seize him, informing him that the Philistines had invaded the land, 1-Samuel 23:7. Or the city of Jerusalem, which was fortified both by nature and art, whither he was brought and placed as king, and enjoyed rest from all his enemies round about him, 2-Samuel 5:6. Or this may spiritually design the church of God, which is called a strong city, being built on Christ the Rock, and having salvation for walls and bulwarks, Isaiah 26:1, where the Lord displays his banner of love, makes discoveries of his marvellous kindness, and commands his blessing for evermore. Some render it "as in a strong city" (g), and take the sense to be, that he was safe, through the kindness of God showed to him in the salvation of him, as if he was in a fortified city (h), and this was marvellous in his eyes, as every instance of providential goodness is to the people of God; especially his lovingkindness showed in spiritual things, in choosing them in Christ, saving them by him, regenerating them by his Spirit, and taking them into his family; which love is free and sovereign, distinguishing, unchangeable, from everlasting to everlasting; and so wonderful and astonishing; and which was hid in God until revealed; and sometimes the manifestations of it are withdrawn, and then it is showed again, and fresh discoveries of it are made, and effects of it applied, and the blessings of it bestowed, which occasion thankfulness.
(g) "ut in civitate munita", Junius & Tremellius, Piscator; Michaelis. (h) Aben Ezra, Kimchi, & Ben Melech in loc.

City - In Keilah: where God wonderfully preserved me.