2 Just as the mountains surround Jerusalem, so the LORD surrounds his people, both now and forever.
When your city is surrounded by mountains the enemy can't get to you. You put guards on the mountains to see the enemy coming and fight them off before they can get over the mountains and get to you. (I think I learned that somewhere....)
But the point is well made. Jerusalem was/is surrounded by mountains. Those who are the LORD's people are surrounded by Him! He is their protection. He fights off the enemy.
This is a continuation of what we've already read in the Psalms just a few days ago.
1 I lift up my eyes to the mountains— where does my help come from?
2 My help comes from the LORD, the Maker of heaven and earth.