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 that 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 the 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 means properly bastions, or ramparts. The original means properly a pomoerium, or antemural defense; a space without the wall of a city 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 that they could retreat within the walls.