In that day shall this song be sung in the land of Judah; We have a strong city; salvation will God appoint for walls and bulwarks.
As the overthrow of the apostate faction is described in the close of Isaiah 25:1-12, so the peace of the faithful is here described under the image of a well-fortified city. The whole context, connected as it is with Christ's coming to destroy death (Isaiah 25:8), cannot admit of the restoration from Babylon under Cyrus being its ultimate and exhaustive fulfillment.
In that day - the day of the Lord's appearing (Isaiah 25:6-9).
Shall this song be sung in the land of Judah. Therefore the Jews shall have been by that time restored to their own "land."
Strong city - Jerusalem, strong in Yahweh's protection: type of the new Jerusalem (Psalms 48:1-3), contrasted with the overthrow of "the lofty city" of the ungodly foe, (Isaiah 26:4-7; Isaiah 12:1-6; Isaiah 13:1-22; Isaiah 14:1-32; Revelation 21:10-27; Revelation 22:2; Revelation 22:10-12, etc.)
Salvation will (God) appoint (for) walls and bulwarks - (Isaiah 60:18; Isaiah 33:20-21; Jeremiah 3:23; Zechariah 2:5.)
Bulwarks - the trench with the antemural earth-works exterior to the wall.