Albert Barnes Commentary Isaiah 26:1

"In that day shall this song be sung in the land of Judah: we have a strong city; salvation will he appoint for walls and bulwarks." — Isaiah 26:1 (ASV)

In that day shall this song be sung – by the people of God, on their restoration to their own land.

We have a strong city – Jerusalem. This does not mean that it was then strongly fortified, but that God would guard it, and thus it would be strong. Jerusalem was easily capable of being strongly fortified (Psalms 25:2); but the idea here is that Yahweh would be a protector, and that this would constitute its strength.

Salvation will God appoint for walls – That is, he will himself be the defender of his people in place of walls and bulwarks. A similar expression occurs in Isaiah 60:18 (see also Jeremiah 3:23, and Zechariah 2:5).

Bulwarks – This word properly means bastions or ramparts. The original word properly means a pomoerium, or antemural defense: a space outside the city wall, raised up like a small wall. The Syriac renders it Bar shuro – ‘Son of a wall,’ meaning a small wall. It was usually a breastwork, or heap of earth thrown up around the city, that constituted an additional defense, so that if they were driven from it, they could retreat within the walls.