The city refers to Zion, the eternal city of God, where the Lord is present to protect and bless his people (see Isaiah 1:8, 27; see also Psalm 46). Zion stands in direct contrast with the "fortified towns" of the nations (Isaiah 25:2; 26:5), which are helpless and reduced to "a heap of rubble."
The walls of God's salvation protectively enclose his people. No one can harm them again.