Psalm 87 is about the glorious city of Zion, the ultimate perfect place to live. The ruler of his city welcomes us to it.
We don't think to the earthly city of Jerusalem, though that was what the people of the old testament sometimes meant it as. This is pointing to the eternal kingdom, who's architect and builder is God.
Through Christ we are offered citizenship to this city.
Cities are formed w/ distinct boundaries (walls). The walls were built to protect the citizens.
The city of Zion has its foundations in the holy mountains (very well protected by the mountains, the mountains are this city's walls)
and its founder is the most High God. (God Himself will keep the people of this city safe).
God's mission is to bring the diverse people from all over the world together, even His enemies.
This magnifies the extent of God's grace. The ticket into this city has nothing to do with our past, how good or bad it is, but solely by the grace of God.
God isn't a distant King, He wants to be at the gates writing down the names and welcoming into the city the people He loves. He is a loving King.
In this city, every thirst is quenched, there is nothing lacking.
There are 2 cities where everyone is going. Zion, or another city.
In this other city, there is only instability, brokenness, division, hatred. In this other city, the people are hated and judged by God. In this other city, the people seek satisfaction but never find it.