In Ancient times the people built huge walls around the cities to protect them from outside enemies. While these walls stood tall and the watchmen stood guard to keep opponents away, many of these cities boasted much destruction from the inside out.
A city that is not God-centered will destroy itself.
“Unless the Lord builds the house, its builders labor in vain!” Psalm 127:1.
A life needs to be God-centered so that it is protected from the inside out.