If you have ever visited the remains of an ancient walled city you can imagine the feeling. There is a sense of enclosure and safety. There is a sense of compactness. Narrow streets are filled with shops and stalls teeming with commerce and life. There is a sense of being in the center of the rest of the world.
People speak in every sort of strange tongue and the air is full of every sort of exotic fragrance.
In a world full of dangers and calamities, Isaiah and the Psalmist try to imagine what the Lord has planned, and they picture a safe walled city, compact and at unity with itself. They picture a city to which all the people of the earth will stream and be at peace in the safety of one wall. This is Jerusalem. Not the Jerusalem that is, but the Jerusalem that is meant to be. This is the promised Jerusalem.