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. We have a strong city - In opposition to the city of the enemy, which God hath destroyed, Isaiah 25:1-12 (note). See the note there.
Salvation - for walls and bulwarks - חומת וחל chomsoth vachel, walls and redoubts, or the walls and the ditch. חל chel properly signifies the ditch or trench without the wall; see Kimchi. The same rabbin says, This song refers to the time of salvation, i.e., the days of the Messiah.